diff --git a/artemis/agents/flash/runner.py b/artemis/agents/flash/runner.py
index 5a8f722..081ec28 100644
--- a/artemis/agents/flash/runner.py
+++ b/artemis/agents/flash/runner.py
@@ -18,9 +18,9 @@ Executes autonomous reactive mobile workflows through OpenAI Responses.
 
 Conversation shape (shared with the Pro operator, history redesign §3.2):
 the prompt is built every turn from a session :class:`TranscriptLedger` —
-a byte-stable system prefix, the committed earlier turns (append-only; old UI
-lists stripped at depth 1, screenshots resolved to visual summaries at depth
-K, long spans chunk-compressed), and a fresh tail carrying the current
+a byte-stable system prefix, the committed earlier turns (append-only within
+a context phase; old observations and long spans compacted together only at
+pressure thresholds), and a fresh tail carrying the current
 observation under a ``# CURRENT OBSERVATION [T+mm:ss]`` header. Actions run
 synchronously and their tool messages already carry the outcome; only a turn
 whose action failed ends with an ``--- Action Execution Result (T+mm:ss) ---``
@@ -30,7 +30,7 @@ session recording.
 
 Per-turn notices on the tail (the reasoning reminder after a silent turn, the
 no-tool-call nudge, the final-turn warning, the user-guidance wrapper) are
-marked ephemeral: the ledger's scrub edge deletes them once the message leaves
+marked ephemeral: the ledger's compaction pass deletes them when it rewrites
 the recent window, so they never reach the frozen history or a chunk capsule.
 The verbatim body of a user instruction is the exception: it stays in the
 active window until its turn is chunked, whose user lines then carry it on.
@@ -83,7 +83,12 @@ from artemis.llm.router import ModelEndpoint
 from artemis.llm.structured import ParseFailure, parse_structured
 from artemis.mcp.action_executor import McpActionExecutor
 from artemis.mcp.observation import DeviceObservationError, require_observation
-from artemis.memory.transcript import PRO_UI_LIST_MARKER, TranscriptLedger, mark_ephemeral
+from artemis.memory.transcript import (
+    OBSERVATION_SCOPE_RULE,
+    PRO_UI_LIST_MARKER,
+    TranscriptLedger,
+    mark_ephemeral,
+)
 from artemis.services.llm import (
     RobustChatModelWrapper,
     acomplete,
@@ -272,6 +277,8 @@ class FlashRunner:
             image_scrub_depth_relaxed=getattr(cfg, "image_scrub_depth_relaxed", None),
             context_budget_tokens=getattr(cfg, "context_budget_tokens", None),
             start_ratio=getattr(cfg, "start_ratio", None),
+            soft_ratio=getattr(cfg, "soft_ratio", 0.7),
+            hard_ratio=getattr(cfg, "hard_ratio", 0.9),
             session_start=session_start if isinstance(session_start, (int, float)) else None,
         )
         if engine is not None:
@@ -1247,7 +1254,13 @@ class FlashRunner:
         """Installs the static prefix and captures the initial device state."""
         ledger = self._build_ledger()
         ledger.set_static_prefix(
-            [SystemMessage(content=self._render_system_prompt(tools_declaration))]
+            [
+                SystemMessage(
+                    content=self._render_system_prompt(tools_declaration)
+                    + "\n"
+                    + OBSERVATION_SCOPE_RULE
+                )
+            ]
         )
 
         # Capture Initial State (Screenshot + UI Tree)
@@ -1369,7 +1382,7 @@ class FlashRunner:
             )
             pending_notices = []
 
-            # S + F + A + tail: chunk compression and the scrub edge advance here.
+            # S + F + A + tail: history changes only at a compaction boundary.
             messages = ledger.render([tail])
             turn_base = len(messages) - 1
             current_tools = report_only_tools if is_final else self._get_tools()
@@ -1427,7 +1440,7 @@ class FlashRunner:
             # ratio — only from a provider measurement, never from the
             # estimate (same as the Pro operator).
             ledger.record_prompt_tokens(
-                step_token_usage.get("prompt_tokens"),
+                step_token_usage.get("prompt_tokens") if measured else None,
                 messages=messages[:-1] if measured else None,
             )
 
diff --git a/artemis/agents/operator/operator.py b/artemis/agents/operator/operator.py
index 0913b0a..c81d787 100644
--- a/artemis/agents/operator/operator.py
+++ b/artemis/agents/operator/operator.py
@@ -330,6 +330,8 @@ class OperatorNode:
             image_scrub_depth_relaxed=getattr(cfg, "image_scrub_depth_relaxed", None),
             context_budget_tokens=getattr(cfg, "context_budget_tokens", None),
             start_ratio=getattr(cfg, "start_ratio", None),
+            soft_ratio=getattr(cfg, "soft_ratio", 0.7),
+            hard_ratio=getattr(cfg, "hard_ratio", 0.9),
             session_start=session_start if isinstance(session_start, (int, float)) else None,
         )
         # L2/L3 chunk compression (M3) rides the same flag; without a
@@ -392,7 +394,7 @@ class OperatorNode:
         """
         from langchain_core.messages import HumanMessage
 
-        from artemis.memory.transcript import mark_ephemeral
+        from artemis.memory.transcript import OBSERVATION_SCOPE_RULE, mark_ephemeral
 
         ledger = self._ensure_transcript_ledger(state)
 
@@ -407,6 +409,7 @@ class OperatorNode:
             if getattr(self.ctx, "plugins", None):
                 prefix.append(self.ctx.plugins.prompt())
             prefix.append(static_text)
+            prefix.append(OBSERVATION_SCOPE_RULE)
             ledger.set_static_prefix([SystemMessage(content="\n".join(prefix))])
 
         # 2. F region cold start: an empty ledger over an existing step record
@@ -496,7 +499,7 @@ class OperatorNode:
                 tail_content.append(part)
         tail = HumanMessage(content=tail_content)
         # Per-turn notices (reminders, hints, guidance, incident blocks) are
-        # deleted by the scrub edge once the turn leaves the recent window.
+        # deleted only when phase compaction rewrites the recent window.
         if builder.ephemeral_indices:
             mark_ephemeral(tail, builder.ephemeral_indices)
 
@@ -635,12 +638,8 @@ class OperatorNode:
                     transcript_ledger.record_prompt_tokens(
                         usage.get("prompt_tokens"), messages=current_messages
                     )
-            elif hasattr(transcript_ledger, "record_prompt_tokens"):
-                # No usage metadata from the provider: record an estimate so
-                # the start gate and thresholds still see a context base.
-                from artemis.memory.transcript import estimate_prompt_tokens
-
-                transcript_ledger.record_prompt_tokens(estimate_prompt_tokens(current_messages))
+            # Without provider usage the ledger estimates the complete next
+            # prompt at render time; do not label that estimate as a measurement.
 
             if response.content:
                 if isinstance(response.content, str):
diff --git a/artemis/config/agent.py b/artemis/config/agent.py
index c8ce7ab..906bdbd 100644
--- a/artemis/config/agent.py
+++ b/artemis/config/agent.py
@@ -425,7 +425,7 @@ class MemoryRuntimeConfig(BaseModel):
 
 
 class MemoryTranscriptConfig(BaseModel):
-    """Scrub-edge options for the message transcript (agent.memory.transcript)."""
+    """Phased context compaction for the shared Flash/Pro transcript."""
 
     enabled: bool = Field(
         default=True,
@@ -443,7 +443,7 @@ class MemoryTranscriptConfig(BaseModel):
         default=3,
         ge=1,
         description=(
-            "Depth K of the screenshot scrub edge from the start gate up: the"
+            "At a compaction boundary, depth K of the screenshot scrub edge: the"
             " K-th most recent screenshot (the live one counts as depth 1) is"
             " replaced by its visual summary."
         ),
@@ -452,11 +452,9 @@ class MemoryTranscriptConfig(BaseModel):
         default=6,
         ge=1,
         description=(
-            "Screenshot scrub depth while the operator's measured context is"
-            " below budget*start_ratio (nothing is being replaced yet, so"
-            " more raw screenshots stay in view). Must be >= image_scrub_depth;"
-            " set equal to it to disable the occupancy-driven depth. A change"
-            " of depth never rewrites an already-scrubbed message."
+            "Screenshot depth for an explicit low-occupancy compaction. Normal"
+            " turns retain previously sent images unchanged. Must be >="
+            " image_scrub_depth."
         ),
     )
     pending_grace_steps: int = Field(
@@ -472,13 +470,10 @@ class MemoryTranscriptConfig(BaseModel):
         default=1,
         ge=1,
         description=(
-            "Text edge of the scrub: once a message is deeper than this many"
-            " image-bearing observations (1 = as soon as a newer observation"
-            " exists) its UI element list, plan recitation and per-turn"
-            " ephemeral blocks are removed. Kept shallow on purpose: only the"
-            " live observation carries an indexed element list, so a stale"
-            " [n] index can never be picked up as a target. Screenshots follow"
-            " image_scrub_depth / image_scrub_depth_relaxed."
+            "At phase compaction, remove old UI lists, plan recitations and"
+            " ephemeral notices beyond this observation depth. Between phases"
+            " old content stays immutable; only the latest observation may"
+            " supply current action targets."
         ),
     )
     context_budget_tokens: int = Field(
@@ -496,12 +491,9 @@ class MemoryTranscriptConfig(BaseModel):
         ge=0.0,
         lt=1.0,
         description=(
-            "Start gate: below budget*start_ratio of the operator's measured"
-            " prompt size, closed segments only prepare their capsule in the"
-            " background — the original turns stay in the transcript. Ready"
-            " chunks replace their turns once the measured context reaches"
-            " this ratio (the hard threshold overrides it). 0 restores"
-            " swap-as-soon-as-ready."
+            "Preparation gate: at budget*start_ratio begin background chunk"
+            " summaries without replacing the live prefix. Zero enables early"
+            " preparation, not immediate replacement. Swaps wait for soft_ratio."
         ),
     )
     soft_ratio: float = Field(
@@ -509,9 +501,9 @@ class MemoryTranscriptConfig(BaseModel):
         gt=0.0,
         lt=1.0,
         description=(
-            "Soft threshold: when the operator's last measured prompt size"
-            " reaches budget*soft_ratio, the oldest open segment is"
-            " chunk-compressed (L2)."
+            "At budget*soft_ratio, replace prepared summaries as one batch and"
+            " scrub old observations, starting a new stable context phase."
+            " Pending summaries retain original turns until ready or hard pressure."
         ),
     )
     hard_ratio: float = Field(
@@ -519,7 +511,7 @@ class MemoryTranscriptConfig(BaseModel):
         gt=0.0,
         le=1.0,
         description=(
-            "Hard threshold: when the operator's last measured prompt size"
+            "Hard threshold: when the projected current prompt size"
             " reaches budget*hard_ratio, ready chunks swap as L2 blocks first;"
             " only if that would not bring the estimated context back under the"
             " line does everything closed force-swap (pending chunks included)"
diff --git a/artemis/context.py b/artemis/context.py
index 06fd460..b0aa7a5 100644
--- a/artemis/context.py
+++ b/artemis/context.py
@@ -267,6 +267,9 @@ class ArtemisContext(BaseModel):
     operator's transcript prompt path (``agent.memory.transcript.enabled``).
     Lives beside ``step_memory``; the State never carries message payloads."""
 
+    prompt_prefix_tracker: Any | None = None
+    """Task-local, hash-only prefix diagnostics shared by bound LLM wrappers."""
+
     async def __aenter__(self) -> ArtemisContext:
         return self
 
diff --git a/artemis/llm/prefix_cache.py b/artemis/llm/prefix_cache.py
new file mode 100644
index 0000000..4b293e4
--- /dev/null
+++ b/artemis/llm/prefix_cache.py
@@ -0,0 +1,65 @@
+"""Local prefix diagnostics; retain hashes only, never prompt/image payloads.
+
+These compare adapted messages, not the gateway's tokenized wire prefix.
+Only provider-reported cached_tokens is a measured cache hit.
+"""
+
+import hashlib
+import json
+
+from langchain_core.messages import BaseMessage
+
+
+def _digest(value) -> str:
+    return hashlib.sha256(
+        json.dumps(value, ensure_ascii=False, sort_keys=True, default=str).encode("utf-8")
+    ).hexdigest()
+
+
+class PromptPrefixTracker:
+    def __init__(self):
+        self._previous: dict[str, tuple[str, tuple[str, ...]]] = {}
+
+    def observe(self, identity: str, messages, options: dict) -> dict:
+        identity = _digest(identity)
+        if not isinstance(messages, (list, tuple)) or not all(
+            isinstance(message, BaseMessage) for message in messages
+        ):
+            return {"comparison": "unavailable"}
+        fingerprints = tuple(
+            _digest(
+                {
+                    "role": message.type.removesuffix("MessageChunk").lower(),
+                    "content": message.content,
+                    "name": message.name,
+                    "tool_calls": getattr(message, "tool_calls", None),
+                    "tool_call_id": getattr(message, "tool_call_id", None),
+                }
+            )
+            for message in messages
+        )
+        options_hash = _digest(options)
+        previous = self._previous.get(identity)
+        # Task-scoped and bounded even if a caller tries many endpoint roles.
+        if identity not in self._previous and len(self._previous) >= 32:
+            self._previous.pop(next(iter(self._previous)))
+        self._previous[identity] = (options_hash, fingerprints)
+        common = 0
+        if previous:
+            for old, new in zip(previous[1], fingerprints):
+                if old != new:
+                    break
+                common += 1
+        schema_changed = previous is not None and previous[0] != options_hash
+        return {
+            "comparison": "local_message_fingerprints",
+            "message_count": len(fingerprints),
+            "previous_message_count": len(previous[1]) if previous else None,
+            "shared_prefix_messages": common if previous else None,
+            "first_changed_message": (
+                common + 1 if previous and common < len(previous[1]) else None
+            ),
+            "append_only": bool(previous and common == len(previous[1]) and not schema_changed),
+            "tools_or_format_changed": schema_changed,
+            "cold_start": previous is None,
+        }
diff --git a/artemis/memory/chunking.py b/artemis/memory/chunking.py
index a61c226..3d95f58 100644
--- a/artemis/memory/chunking.py
+++ b/artemis/memory/chunking.py
@@ -1074,7 +1074,12 @@ class HistoryChunkManager:
         soft = base_tokens is not None and base_tokens >= self._budget * self._soft_ratio
         hard = base_tokens is not None and base_tokens >= self._budget * self._hard_ratio
 
-        closed_any = self._close_new_segments(ledger, soft, hard)
+        preparing = base_tokens is not None and base_tokens >= self._budget * self._start_ratio
+        closed_any = (
+            self._close_new_segments(ledger, soft, hard)
+            if hard or (preparing and (not soft or not self._awaiting))
+            else False
+        )
         if closed_any or soft or hard:
             # Failure rung of the degradation ladder: exhausted capsules are
             # re-dispatched (the lens retries a fallback model per attempt);
@@ -1206,14 +1211,10 @@ class HistoryChunkManager:
         contiguous READY prefix may swap — an older pending segment keeps its
         (and every younger segment's) original text live.
 
-        Start gate: closing a segment (milestone/size triggers) only *prepares*
-        its capsule in the background; the original text is replaced only once
-        the operator's measured context has reached ``budget * start_ratio``.
-        Below that the ready chunks are held in the awaiting queue, so at low
-        occupancy the model keeps reading the full transcript while capsules
-        are already on hand for when pressure arrives. An unknown base (no
-        measured operator call yet, or a provider without usage metadata)
-        cannot gate and falls through to swap-on-ready.
+        The start gate enables background preparation. Only the soft gate
+        replaces live history, in one batch after all prepared headers are
+        ready. The hard gate may force a swap if summaries are still pending.
+        Missing provider usage uses the ledger's current prompt estimate.
 
         Hard threshold ladder (L2 first, L3 last): at ``budget * hard_ratio``
         the ready prefix swaps as ordinary L2 chunk blocks when the estimated
@@ -1262,7 +1263,13 @@ class HistoryChunkManager:
             else:
                 swap = list(self._awaiting)  # rung 2: force everything, fold
                 fold = True
-        elif base_tokens is not None and base_tokens < self._budget * self._start_ratio:
+        elif base_tokens is None or base_tokens < self._budget * self._soft_ratio:
+            if base_tokens is not None:
+                self._announce_held(base_tokens, ledger)
+            return
+        elif len(ready_prefix) < len(self._awaiting):
+            # Batch the prepared summaries into one rewrite. Async completion
+            # must not cause a succession of partial prefix replacements.
             self._announce_held(base_tokens, ledger)
             return
         else:
@@ -1290,7 +1297,7 @@ class HistoryChunkManager:
         # merged into the period paragraph); a pending chunk has no summary
         # size to report, so its trace carries no compression ratio.
         swapped = [e for e in swap if e["chunk"] is not None]
-        source_tokens_total = 0
+        source_tokens_total = sum(ledger.turn_prompt_tokens(e["turns"]) for e in swap)
         replacement_tokens_total = 0
         for entry in swapped:
             chunk = entry["chunk"]
@@ -1298,7 +1305,6 @@ class HistoryChunkManager:
                 chunk.summary_chars = len(render_chunk_block(chunk))
             else:
                 chunk.summary_chars = None
-            source_tokens_total += self._chars_to_tokens(chunk.source_chars, ledger)
             replacement_tokens_total += self._chars_to_tokens(
                 self._replacement_chars(chunk, folded=fold), ledger
             )
@@ -1392,18 +1398,17 @@ class HistoryChunkManager:
         delta = 0
         for entry in entries:
             chunk = entry["chunk"]
-            if chunk is None:
-                continue
-            delta += self._chars_to_tokens(
-                self._replacement_chars(chunk), ledger
-            ) - self._chars_to_tokens(chunk.source_chars, ledger)
+            replacement = self._replacement_chars(chunk) if chunk is not None else 0
+            delta += self._chars_to_tokens(replacement, ledger) - ledger.turn_prompt_tokens(
+                entry["turns"]
+            )
         return max(0, int(base_tokens) + delta)
 
     def _announce_held(self, base_tokens: int, ledger: Any = None) -> None:
-        """Once per chunk: its capsule is ready but the start gate holds the
+        """Once per chunk: its capsule is ready but the soft gate holds the
         original text live. The timeline line stays ``running`` with a
         ``held`` note so the UI can say the summary is ready and waiting."""
-        threshold = int(self._budget * self._start_ratio)
+        threshold = int(self._budget * self._soft_ratio)
         for entry in self._awaiting:
             chunk = entry["chunk"]
             if chunk is None or chunk.status != "ready" or entry.get("held_announced"):
@@ -1468,7 +1473,7 @@ class HistoryChunkManager:
         """The live context base for the start/soft/hard thresholds.
 
         An injected ``meter_getter`` wins (tests / custom wiring); otherwise
-        the base is the owning operator's last measured prompt size recorded
+        the base is the ledger's projected current prompt, calibrated from usage recorded
         on the ledger (:meth:`TranscriptLedger.record_prompt_tokens`). The
         session-wide token meter is deliberately NOT consulted: its
         ``last_prompt_tokens`` is overwritten by whichever agent called last
@@ -1481,6 +1486,9 @@ class HistoryChunkManager:
             except Exception:
                 return None
         try:
+            projected = getattr(ledger, "projected_prompt_tokens", None)
+            if isinstance(projected, int) and projected > 0:
+                return projected
             last = getattr(ledger, "last_prompt_tokens", None)
             return int(last) if last else None
         except (TypeError, ValueError):
diff --git a/artemis/memory/transcript.py b/artemis/memory/transcript.py
index e7fadc9..ac2a7b2 100644
--- a/artemis/memory/transcript.py
+++ b/artemis/memory/transcript.py
@@ -24,14 +24,11 @@ Messages are organized into four regions:
   (tool_calls and native thinking preserved by reference), the in-turn
   ToolMessages, and the turn's result message (every turn on the Pro path;
   only when an action failed on the Flash path). The
-  scrub edge (:class:`~artemis.agents.flash.context_compressor.ScrubEdgeCompressor`)
-  has two depths: a shallow text edge (UI list, plan recitation and ephemeral
-  per-turn blocks leave as soon as a newer observation exists) and a
-  screenshot edge that follows the occupancy tier (relaxed below the start
-  gate, tightened past it), where the screenshot is replaced in place by its
-  visual summary. Every rewrite happens once and is never undone; everything
-  newer is byte-identical to what the model was first shown. Messages are
-  never removed or reordered, so tool-call/response pairs are never split.
+  scrub edge runs only at a pressure-driven phase boundary, together with
+  chunk replacement. Normal renders preserve all previously sent content,
+  including screenshots and UI lists. Chunk summaries use a compact copy of
+  each turn; their background completion never rewrites the live prefix.
+  Tool-call/response pairs are never split.
 - **T (current tail)**: built fresh every turn by the operator; passed to
   :meth:`render` and only enters A when the turn is committed.
 
@@ -74,8 +71,8 @@ RESTORED_HISTORY_HEADER = "[Restored history]"
 
 #: ``HumanMessage.additional_kwargs`` key listing the indices of content blocks that
 #: are only meaningful for the turn they were built for (reminders, hints, user
-#: guidance, per-turn notices). The scrub edge deletes them at its text edge, so
-#: they never reach the frozen region or a chunk capsule. The indices always
+#: guidance, per-turn notices). At a compaction boundary the scrub edge deletes them;
+#: capsule source copies also omit them. The indices always
 #: describe the message's *current* block layout: a rewrite that removes blocks
 #: before the text edge (the screenshot swap when the image edge is shallower)
 #: remaps the surviving indices in place, and the key is dropped once the blocks
@@ -83,6 +80,13 @@ RESTORED_HISTORY_HEADER = "[Restored history]"
 #: marking costs the model nothing.
 EPHEMERAL_BLOCKS_KEY = "ephemeral_blocks"
 
+OBSERVATION_SCOPE_RULE = (
+    "历史截图、元素编号、坐标和逐轮提醒仅用于回顾。每次操作必须依据同一设备/来源的"
+    "最新观察；旧消息中的 CURRENT OBSERVATION 不表示仍然有效。最新任务计划和用户指导"
+    "优先于旧版本；旧的结束提示不得覆盖新指导。需要确认当前状态时重新观察，禁止从旧"
+    "元素列表选择目标或用历史截图证明当前操作成功。"
+)
+
 
 def mark_ephemeral(message: BaseMessage, indices) -> None:
     """Flag content blocks of ``message`` (by index) as ephemeral, see
@@ -190,6 +194,10 @@ def render_turn_transcript(messages: list[BaseMessage]) -> str:
 #: (Gemini's flat per-image cost; the Flash runner uses the same figure).
 IMAGE_BLOCK_TOKENS = 258
 
+# Conservative growth allowance for a new screenshot before provider usage
+# arrives. Removed images use IMAGE_BLOCK_TOKENS to avoid overstating savings.
+PROJECTED_IMAGE_TOKENS = 2048
+
 #: Characters per token assumed before any calibrated measurement.
 DEFAULT_CHARS_PER_TOKEN = 4.0
 
@@ -268,8 +276,10 @@ class TranscriptLedger:
         pending_grace_steps: int = 3,
         xml_scrub_depth: int | None = None,
         image_scrub_depth_relaxed: int | None = None,
-        context_budget_tokens: int | None = None,
+        context_budget_tokens: int | None = 80000,
         start_ratio: float | None = None,
+        soft_ratio: float = 0.7,
+        hard_ratio: float = 0.9,
         clock: Callable[[], float] | None = None,
         session_start: float | None = None,
     ):
@@ -308,6 +318,10 @@ class TranscriptLedger:
         # calls sharing the session can never masquerade as the operator's
         # context (see HistoryChunkManager._context_base_tokens).
         self._last_prompt_tokens: int | None = None
+        self._prompt_sample: tuple[int, int, int] | None = None
+        self._projected_prompt_tokens: int | None = None
+        self.context_phase = 0
+        self.last_compaction: dict[str, Any] | None = None
         # Session-calibrated characters-per-token ratio (see
         # record_prompt_tokens): the chunker converts transcript characters
         # to tokens through it, so CJK-heavy sessions (~1 char/token on
@@ -326,26 +340,21 @@ class TranscriptLedger:
         # it to decide whether to attach the shared reasoning reminder.
         self._last_turn_silent: bool = False
 
-        # Text edge (default 1): UI list, plan recitation and ephemeral blocks
-        # leave as soon as a newer observation exists; screenshots follow K.
+        # At phase compaction, strip old UI/plan/notices and resolve images.
+        # Between boundaries none of these fields is rewritten.
         self._xml_scrub_depth = xml_scrub_depth
 
-        # Occupancy-driven screenshot depth. Below the start gate
-        # (``last_prompt_tokens < budget * start_ratio``, or no measurement
-        # yet) the scrub edge sits at the relaxed depth so the model keeps
-        # more raw screenshots while nothing is being replaced; from the
-        # start gate up it tightens to ``image_scrub_depth``. Scrubbing is
-        # irreversible, so a change of depth never rewrites a message:
-        # tightening scrubs the extra depths once, relaxing lets new images
-        # live longer. See :attr:`effective_image_scrub_depth`.
+        # Depth settings apply only inside an explicit compaction pass.
         self._image_scrub_depth = max(1, int(image_scrub_depth))
         self._image_scrub_depth_relaxed = (
             max(self._image_scrub_depth, int(image_scrub_depth_relaxed))
             if image_scrub_depth_relaxed is not None
             else self._image_scrub_depth
         )
-        self._context_budget_tokens = int(context_budget_tokens) if context_budget_tokens else None
+        self._context_budget_tokens = int(context_budget_tokens or 80000)
         self._start_ratio = float(start_ratio) if start_ratio is not None else None
+        self._soft_ratio = float(soft_ratio)
+        self._hard_ratio = float(hard_ratio)
 
         # id(message) -> summary-job key (DataEngine step id). A side map keeps
         # the key out of the serialized message payload entirely.
@@ -356,7 +365,8 @@ class TranscriptLedger:
         # whose failure depends on which side is imported first.
         from artemis.agents.flash.context_compressor import ScrubEdgeCompressor
 
-        self._compressor = ScrubEdgeCompressor(
+        self._compressor_type = ScrubEdgeCompressor
+        self._compressor_options = dict(
             summarizer=step_memory,
             prune_history_xml=prune_history_xml,
             image_scrub_depth=image_scrub_depth,
@@ -366,6 +376,7 @@ class TranscriptLedger:
             strip_markers=(PRO_UI_LIST_MARKER, PLAN_RECITATION_MARKER),
             tail_offset=1,
         )
+        self._compressor = ScrubEdgeCompressor(**self._compressor_options)
 
     # ------------------------------------------------------------------
     # Session clock
@@ -388,6 +399,27 @@ class TranscriptLedger:
         first measured call (or when the provider reports no usage)."""
         return self._last_prompt_tokens
 
+    @property
+    def projected_prompt_tokens(self) -> int | None:
+        """Current prompt including the fresh tail, estimated before compaction."""
+        return self._projected_prompt_tokens
+
+    def estimate_context_tokens(self, messages: list[BaseMessage]) -> int:
+        chars, images = measure_prompt_content(messages)
+        if self._prompt_sample is None:
+            # Without provider usage, count CJK conservatively rather than //4.
+            return max(1, self._last_prompt_tokens or 0, chars + images * PROJECTED_IMAGE_TOKENS)
+        prev_chars, prev_images, tokens = self._prompt_sample
+        return max(
+            1,
+            tokens
+            + int((chars - prev_chars) / self._chars_per_token)
+            # New screenshots may tile to thousands of tokens. For removals,
+            # use the smaller lower bound rather than overclaiming savings.
+            + (images - prev_images)
+            * (PROJECTED_IMAGE_TOKENS if images >= prev_images else IMAGE_BLOCK_TOKENS),
+        )
+
     @property
     def last_turn_silent(self) -> bool:
         """True when the last committed turn had no visible reasoning text.
@@ -455,7 +487,7 @@ class TranscriptLedger:
 
     @property
     def effective_image_scrub_depth(self) -> int:
-        """The screenshot depth the scrub edge uses at the next render: the
+        """The screenshot depth used at the next compaction boundary: the
         relaxed depth below the start gate, the tight depth from there up."""
         if self.below_start_gate:
             return self._image_scrub_depth_relaxed
@@ -514,6 +546,7 @@ class TranscriptLedger:
         if value <= 0:
             return
         self._last_prompt_tokens = value
+        self._projected_prompt_tokens = None
         if messages is None:
             return
         try:
@@ -521,6 +554,7 @@ class TranscriptLedger:
         except Exception as exc:
             logger.debug(f"Prompt calibration skipped: {exc}", exc_info=True)
             return
+        self._prompt_sample = (text_chars, image_count, value)
         if text_chars <= 0:
             return
 
@@ -763,11 +797,24 @@ class TranscriptLedger:
         return sum(len(self.turn_transcript(turn)) for turn in turns)
 
     def turn_transcript(self, turn: dict) -> str:
-        """Plain-text rendering of one committed turn exactly as it currently
-        stands in the active region (i.e. after the scrub edge), see
-        :func:`render_turn_transcript`. Chunk compression feeds this to the
-        segment capsule so the capsule digests precisely what it replaces."""
-        return render_turn_transcript(self._active[turn["start"] : turn["end"]])
+        """Compact capsule source without rewriting the live model prefix."""
+        originals = self._active[turn["start"] : turn["end"]]
+        copies = [message.model_copy(deep=True) for message in originals]
+        keys = {id(new): self._step_keys.get(id(old)) for old, new in zip(originals, copies)}
+        options = {
+            **self._compressor_options,
+            "pending_grace_steps": 0,
+            "summary_key_getter": lambda message: keys.get(id(message)),
+        }
+        self._compressor_type(**options).compress(copies, image_scrub_depth=1)
+        return render_turn_transcript(copies)
+
+    def turn_prompt_tokens(self, turns: list[dict]) -> int:
+        """Size actually removed by a swap, including retained image blocks."""
+        chars, images = measure_prompt_content(
+            [message for turn in turns for message in self._active[turn["start"] : turn["end"]]]
+        )
+        return self.chars_to_tokens(chars) + images * IMAGE_BLOCK_TOKENS
 
     def freeze_turns(self, turn_count: int, frozen_blocks: list[BaseMessage]) -> None:
         """Compression event: consume the oldest ``turn_count`` unchunked turns
@@ -778,6 +825,7 @@ class TranscriptLedger:
         """
         if turn_count <= 0:
             self._frozen_blocks = list(frozen_blocks)
+            self.context_phase += 1
             return
         available = self._turns[self._chunked_turn_count :]
         if turn_count > len(available):
@@ -788,36 +836,38 @@ class TranscriptLedger:
         self._chunked_turn_count += turn_count
         self._active_start = self._turns[self._chunked_turn_count - 1]["end"]
         self._frozen_blocks = list(frozen_blocks)
+        self.context_phase += 1
 
     # ------------------------------------------------------------------
     # Rendering
     # ------------------------------------------------------------------
 
-    def render(self, tail: list[BaseMessage]) -> list[BaseMessage]:
-        """Advance compression and return ``S + F + A + tail``.
+    def compact_active(self) -> bool:
+        """Scrub old observations only at an explicit context phase boundary.
 
-        Order per turn: the occupancy band is read once (from the operator's
-        last measured prompt), the scrub edge advances over the active window
-        at that band's screenshot depth, then the chunker runs (a triggered
-        compression event deep-mutates the frozen region and advances the F/A
-        boundary). Scrub-before-chunk guarantees that the turns a chunk
-        closes over — all older than the sliding-window floor — have already
-        had their screenshots resolved when the relaxed depth is no deeper
-        than the floor plus one. The returned list is a fresh container:
-        appending to it (the operator's in-turn tool loop) never mutates the
-        ledger regions.
+        Copy messages first: a previously returned model input must remain
+        immutable even when this ledger starts a new phase.
         """
+        active = self._active
+        copies = [message.model_copy(deep=True) for message in active]
+        protected = set()
+        for old, new in zip(active, copies, strict=True):
+            if id(old) in self._step_keys:
+                self._step_keys[id(new)] = self._step_keys.pop(id(old))
+            if id(old) in self._fresh_tool_messages:
+                protected.add(id(new))
         self._compressor.compress(
-            self._active,
+            copies,
             image_scrub_depth=self.effective_image_scrub_depth,
-            protected_message_ids=self._fresh_tool_messages,
+            protected_message_ids=protected,
         )
-        self._fresh_tool_messages.clear()
-        if self._chunker is not None:
-            try:
-                self._chunker.on_render(self)
-            except Exception as e:
-                logger.error(f"History chunker render hook failed: {e}")
+        self._active = copies
+        return any(
+            old.content != new.content or old.additional_kwargs != new.additional_kwargs
+            for old, new in zip(active, copies, strict=True)
+        )
+
+    def _messages(self, tail: list[BaseMessage]) -> list[BaseMessage]:
         return [
             *self._static,
             *self._restored,
@@ -825,3 +875,35 @@ class TranscriptLedger:
             *self._active[self._active_start :],
             *tail,
         ]
+
+    def render(self, tail: list[BaseMessage]) -> list[BaseMessage]:
+        """Append normally; batch history rewrites at pressure-driven boundaries."""
+        before = self.estimate_context_tokens(self._messages(tail))
+        self._projected_prompt_tokens = before
+        phase = self.context_phase
+        if self._chunker is not None:
+            try:
+                self._chunker.on_render(self)
+            except Exception as e:
+                logger.error(f"History chunker render hook failed: {e}")
+        if (
+            self.context_phase != phase
+            or (self._chunker is None and before >= self._context_budget_tokens * self._soft_ratio)
+            or before >= self._context_budget_tokens * self._hard_ratio
+        ):
+            rewritten = self.compact_active()
+            after = self.estimate_context_tokens(self._messages(tail))
+            if self.context_phase != phase or rewritten:
+                self.context_phase = phase + 1
+                self.last_compaction = {
+                    "phase": self.context_phase,
+                    "reason": "context_pressure",
+                    "before_tokens": before,
+                    "after_tokens": after,
+                    "estimated": True,
+                }
+                logger.info(
+                    f"Context phase {self.context_phase}: {before} -> {after} tokens (estimated)"
+                )
+        self._fresh_tool_messages.clear()
+        return self._messages(tail)
diff --git a/artemis/resources/config/artemis.jsonc b/artemis/resources/config/artemis.jsonc
index 0cc1fbe..ef47fb0 100644
--- a/artemis/resources/config/artemis.jsonc
+++ b/artemis/resources/config/artemis.jsonc
@@ -96,6 +96,7 @@
         "enabled": true,
         // 历史压缩预算，允许范围 1024–300000；上下文参照上限固定为 300k。
         "context_budget_tokens": 80000,
+        // 阶段内仅追加；35% 开始后台准备摘要，70% 成批压缩，90% 紧急折叠。
         "start_ratio": 0.35,
         "soft_ratio": 0.7,
         "hard_ratio": 0.9,
diff --git a/artemis/services/llm.py b/artemis/services/llm.py
index c2194b5..40b8e7d 100644
--- a/artemis/services/llm.py
+++ b/artemis/services/llm.py
@@ -58,6 +58,7 @@ from artemis.llm.reliability import (
     retry_policy_for,
 )
 from artemis.llm.messages import adapt_responses_messages
+from artemis.llm.prefix_cache import PromptPrefixTracker
 from artemis.llm.router import ModelEndpoint, ModelFactory, ModelProvider
 from artemis.services.token_meter import record_llm_usage
 from artemis.llm.usage import usage_from_message
@@ -671,6 +672,27 @@ class RobustChatModelWrapper:
         record_llm_usage(engine, response, source=self._endpoint_key())
 
     def _diagnostics(self, args, kwargs):
+        prefix = None
+        if self.ctx is not None:
+            tracker = getattr(self.ctx, "prompt_prefix_tracker", None)
+            if not isinstance(tracker, PromptPrefixTracker):
+                tracker = PromptPrefixTracker()
+                self.ctx.prompt_prefix_tracker = tracker
+            bound_options = getattr(self.base_model, "kwargs", {})
+            options = {
+                **(bound_options if isinstance(bound_options, dict) else {}),
+                **{
+                    key: kwargs[key]
+                    for key in ("tools", "tool_choice", "response_format")
+                    if key in kwargs
+                },
+            }
+            identity = f"{kwargs.get('prompt_cache_key')}:{self.role}:{self._endpoint_key()}"
+            prefix = tracker.observe(identity, args[0] if args else kwargs.get("input"), options)
+            ledger = getattr(self.ctx, "transcript_ledger", None)
+            if ledger is not None:
+                prefix["context_phase"] = getattr(ledger, "context_phase", 0)
+                prefix["last_compaction"] = getattr(ledger, "last_compaction", None)
         return {
             "provider": self._provider_value(),
             "model": self.endpoint.model_name if self.endpoint else None,
@@ -680,6 +702,7 @@ class RobustChatModelWrapper:
             "is_fallback": self.is_fallback,
             "images": input_images(args, kwargs),
             "prompt_cache_key": kwargs.get("prompt_cache_key"),
+            "prefix_cache": prefix,
         }
 
     async def _invoke_attempt(self, *args, **kwargs):
diff --git a/config/artemis.jsonc b/config/artemis.jsonc
index 0cc1fbe..ef47fb0 100644
--- a/config/artemis.jsonc
+++ b/config/artemis.jsonc
@@ -96,6 +96,7 @@
         "enabled": true,
         // 历史压缩预算，允许范围 1024–300000；上下文参照上限固定为 300k。
         "context_budget_tokens": 80000,
+        // 阶段内仅追加；35% 开始后台准备摘要，70% 成批压缩，90% 紧急折叠。
         "start_ratio": 0.35,
         "soft_ratio": 0.7,
         "hard_ratio": 0.9,
diff --git a/docs/ai-automation.md b/docs/ai-automation.md
index f073a98..160bf7e 100644
--- a/docs/ai-automation.md
+++ b/docs/ai-automation.md
@@ -82,3 +82,19 @@ Flash 和 Pro 默认复用只读 Checker，以任务提示词和用户指导中
 通过 `mobile_config(section="capabilities")` 可读取 `recording` 参数；用户授权后可保存全局默认。单任务用 `runtime_overrides={"recording":{"codec":"h264","computer_scope":"window","max_long_edge":1280,"fps":15}}`，批次在 `task_config.recording` 提供同样对象。JSON 配置的 `agent.recording` 覆盖环境默认；不要把录像参数当作模型截图参数。录屏开关继续独立且默认关闭。
 
 运行信息的上下文参照上限固定为 300,000 tokens，前后端使用相同口径。历史压缩预算默认 80,000，JSON 的 `agent.memory.transcript.context_budget_tokens` 可调但不能超过 300,000；这是压缩触发预算，不是单次生成的输出 token 数，也不是整项任务累计 token 的限额。
+
+
+### 多轮上下文与缓存诊断
+
+Flash、Pro（启用 transcript）和探索共用分阶段上下文。一个阶段内只追加新观察、工具结果和用户指导，保留此前发送的截图、元素列表和消息原文。行动只能使用同一来源的最新观察，旧画面不能证明当前状态。检查器和摘要模型保持独立职责，不混入执行模型的对话。
+
+默认压缩预算为 80,000 tokens：估算占用达到 28,000 后开始后台准备摘要，56,000 后等待本批摘要齐备再一次性替换历史、清理旧观察，开始下一阶段；72,000 时可紧急折叠为带步骤索引的历史。摘要失败时原文保留至硬阈值，原始证据仍可经历史工具读取。触发估算包含本轮新消息和截图，不只看上一轮用量；没有上游 usage 时使用本地保守估算。该预算不是任务累计限额，也不改变 300k 上下文参照上限。
+
+任务 trace 的 `llm_diagnostic` / `llm_request` 在原有 `usage`、耗时和 `prompt_cache_key` 外增加 `prefix_cache`：
+
+- `append_only`、`shared_prefix_messages`：相对同任务、角色和端点上次调用，输入是否仅追加、共享多少完整消息。
+- `first_changed_message`：首个变化消息的位置（从 1 开始）；`tools_or_format_changed` 表示工具或格式参数发生变化。
+- `context_phase`、`last_compaction`：上下文阶段与上次压缩前后的估算 token 数。
+- `cold_start`：该角色/端点在当前进程内没有比较基准；重启后首次请求会重新建立基准。
+
+以上为适配后消息的本地指纹比较，不是网关最终序列化后的 token 前缀，也不是命中率。诊断仅持有哈希，不额外保存提示词、图片正文或凭据。实际缓存率按上游返回的 `cached_tokens / input_tokens` 加权统计，usage 缺失不能当作零命中。首次请求、阶段压缩、模型/工具切换可能降低缓存率；网关的路由、缓存保留时间和实现也会影响结果。该实现使用完整多轮消息，不依赖网关支持 `previous_response_id`，也不承诺固定命中率。
diff --git a/tests/unit/agents/test_flash_runner_ledger.py b/tests/unit/agents/test_flash_runner_ledger.py
index 29c6f8f..828ada9 100644
--- a/tests/unit/agents/test_flash_runner_ledger.py
+++ b/tests/unit/agents/test_flash_runner_ledger.py
@@ -283,13 +283,13 @@ def test_injected_instruction_body_outlives_its_wrapper(mock_context):
     assert guidance.wrapper.startswith(USER_GUIDANCE_MARKER)
     assert "don't send messages" not in guidance.wrapper
 
-    # Scrub edge parity: once the turn has left the live position (a newer
-    # committed observation exists) the wrapper is gone and the body is
-    # still in the committed turn.
+    # Normal turns preserve guidance verbatim; an explicit compaction removes
+    # the old wrapper but preserves the user's standing instruction.
     ledger.stage_turn([tail, AIMessage(content="ok")])
     ledger.commit_staged(step_key="s1")
     ledger.stage_turn([runner._build_tail(ledger, 3, b"IMG", "[1] Send"), AIMessage(content="ok")])
     ledger.commit_staged(step_key="s2")
+    ledger.compact_active()
     rendered = ledger.render([runner._build_tail(ledger, 4, b"IMG", "[1] Send")])
     committed = next(
         m
@@ -679,8 +679,8 @@ async def test_run_calibrates_the_ledger_only_from_measured_usage(mock_context):
     assert record.call_count == 2
     first_call, second_call = record.call_args_list
     assert first_call.args[0] == 1200 and first_call.kwargs["messages"] is not None
-    # The report turn carries no usage metadata: estimated, recorded, not calibrated.
-    assert second_call.args[0] >= 1 and second_call.kwargs["messages"] is None
+    # Missing usage stays unknown; the next render computes its own estimate.
+    assert second_call.args[0] is None and second_call.kwargs["messages"] is None
 
 
 @pytest.mark.asyncio
diff --git a/tests/unit/agents/test_operator_prompt_estimate.py b/tests/unit/agents/test_operator_prompt_estimate.py
index 7367320..30de3fc 100644
--- a/tests/unit/agents/test_operator_prompt_estimate.py
+++ b/tests/unit/agents/test_operator_prompt_estimate.py
@@ -30,7 +30,7 @@ from artemis.agents.operator.operator import OperatorNode
 from artemis.config.agent import MemoryTranscriptConfig
 from artemis.context import ArtemisContext
 from artemis.memory.step_memory import StepMemoryService
-from artemis.memory.transcript import TranscriptLedger, estimate_prompt_tokens
+from artemis.memory.transcript import TranscriptLedger
 
 SCREENSHOT_B64 = base64.b64encode(b"fake-jpeg-bytes").decode("utf-8")
 
@@ -92,7 +92,7 @@ def _llm(captured: list, usage_metadata=None):
 
 
 @pytest.mark.asyncio
-async def test_operator_records_an_estimate_when_the_provider_reports_no_usage():
+async def test_operator_uses_projected_context_without_fabricating_provider_usage():
     ctx = _ctx()
     captured: list = []
     with patch("artemis.agents.operator.operator.get_llm", return_value=_llm(captured)):
@@ -102,8 +102,8 @@ async def test_operator_records_an_estimate_when_the_provider_reports_no_usage()
     ledger = ctx.transcript_ledger
     assert isinstance(ledger, TranscriptLedger)
     assert captured, "the operator made no model call"
-    assert ledger.last_prompt_tokens == estimate_prompt_tokens(captured[-1])
-    assert ledger.last_prompt_tokens > 0
+    assert ledger.last_prompt_tokens is None
+    assert ledger.projected_prompt_tokens > 0
     # No measurement, no calibration.
     assert ledger.chars_per_token == 4.0
 
diff --git a/tests/unit/agents/test_operator_transcript.py b/tests/unit/agents/test_operator_transcript.py
index 91ebd7b..7b7b281 100644
--- a/tests/unit/agents/test_operator_transcript.py
+++ b/tests/unit/agents/test_operator_transcript.py
@@ -355,7 +355,7 @@ async def test_transcript_mode_two_turns_build_four_regions(tmp_path):
         # Text edge at depth 1: the committed observation has lost its plan
         # recitation as soon as the next tail exists (its screenshot stays
         # until the screenshot edge).
-        assert PLAN_RECITATION_MARKER not in str(committed_obs.content)
+        assert PLAN_RECITATION_MARKER in str(committed_obs.content)  # retained until compaction
         # The live tail still recites the plan.
         assert PLAN_RECITATION_MARKER in str(turn2_messages[-1].content)
         # Message layout is S + A + tail.
diff --git a/tests/unit/memory/test_history_chunking.py b/tests/unit/memory/test_history_chunking.py
index 0ba7175..9c5c0ab 100644
--- a/tests/unit/memory/test_history_chunking.py
+++ b/tests/unit/memory/test_history_chunking.py
@@ -192,7 +192,17 @@ def _make(
         capsule_service=capsule,
         # No meter: the base comes from the ledger's operator-recorded prompt
         # size (None until recorded), exactly like production wiring.
-        meter_getter=meter,
+        meter_getter=meter
+        or (
+            lambda: (
+                75_000
+                if chunker.awaiting_chunks
+                and all(
+                    capsule.get_summary(c.capsule_key) is not None for c in chunker.awaiting_chunks
+                )
+                else 40_000
+            )
+        ),
         goal="TEST GOAL",
     )
     ledger.attach_chunker(chunker)
@@ -443,7 +453,8 @@ def test_segment_size_threshold_triggers_without_milestone():
     assert len(chunker.chunks) >= 1
     assert chunker.chunks[0].start_step_number == 1
     total = sum(c.end_step_number - c.start_step_number + 1 for c in chunker.chunks)
-    assert total >= 4
+    assert total >= 3
+    assert chunker.awaiting_chunks  # the next prepared batch has not swapped yet
     assert len(ledger.unchunked_turns()) >= 2  # floor respected
 
 
@@ -458,7 +469,7 @@ def test_source_token_threshold_triggers_chunk():
 
 def test_soft_threshold_compresses_oldest_open_segment():
     steps = [_step(i) for i in range(1, 9)]
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, _, _ = _make(
         steps, min_active=2, max_steps=100, meter=lambda: meter["value"], auto_capsule=True
     )
@@ -843,7 +854,7 @@ def test_hard_threshold_folds_frozen_region_into_recall_only_era_with_step_index
     merged) plus the per-step minimal index of every folded chunk — step,
     session offset and action phrase, results only via search_history."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 13)]
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, _, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
     capsule.resolve("chunk:1-4", _capsule(1, 4))
@@ -1083,7 +1094,7 @@ def test_failed_capsule_redispatches_and_retains_original():
     """Failure rung of the ladder: an exhausted capsule job is re-dispatched at
     the next trigger/pressure render while the original turns stay live."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 9)]
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, _, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
     key = chunker.awaiting_chunks[0].capsule_key
@@ -1106,7 +1117,7 @@ def test_hard_threshold_force_swaps_pending_chunks_into_recall_only_era():
     paragraph degrades to the milestone/step-range list, and ③'s minimal
     index survives for every folded step."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 9)]
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, _, _ = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
     assert chunker.awaiting_chunks[0].status == "pending"
@@ -1150,7 +1161,9 @@ def test_pressure_close_waits_for_min_steps_instead_of_one_turn_chunks():
     _run_turns(ledger, chunker, 7, 8)  # eligible beyond the claim: 1, then 2
     assert [c.capsule_key for c in chunker.awaiting_chunks] == ["chunk:1-3"]
     _run_turns(ledger, chunker, 9, 9)
-    assert [c.capsule_key for c in chunker.awaiting_chunks] == ["chunk:1-3", "chunk:4-6"]
+    assert [c.capsule_key for c in chunker.awaiting_chunks] == [
+        "chunk:1-3"
+    ]  # finish this batch first
 
 
 def test_size_trigger_waits_for_min_steps_even_when_one_turn_exceeds_the_tokens():
@@ -1193,7 +1206,7 @@ def test_start_gate_holds_ready_chunk_below_start_ratio_then_swaps():
     replaced only once the operator's measured context reaches
     budget*start_ratio; until then the ready chunk waits in the queue."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 11)]
-    meter = {"value": 20_000}  # < 0.35 * 100k
+    meter = {"value": 40_000}  # < 0.35 * 100k
     ledger, chunker, engine, _ = _make(
         steps, min_active=2, meter=lambda: meter["value"], auto_capsule=True
     )
@@ -1212,13 +1225,13 @@ def test_start_gate_holds_ready_chunk_below_start_ratio_then_swaps():
     args = held[0]["payload"]["args"]
     assert args["note"] == "held"
     assert (args["context_tokens"], args["context_budget"], args["swap_at_tokens"]) == (
-        20_000,
+        40_000,
         100_000,
-        35_000,
+        70_000,
     )
 
     # Same render cadence, context now past the gate → the ready prefix swaps.
-    meter["value"] = 40_000
+    meter["value"] = 75_000
     _run_turns(ledger, chunker, 9, 9, _hashes(4))
     assert len(chunker.chunks) == 1
     assert chunker.chunks[0].start_step_number == 1
@@ -1226,9 +1239,7 @@ def test_start_gate_holds_ready_chunk_below_start_ratio_then_swaps():
     assert ledger.frozen_blocks != ()
     done = engine.traces[-1]
     assert done["status"] == "success"
-    assert done["payload"]["args"]["context_tokens"] == 40_000 - (
-        chunker.chunks[0].source_chars // 4
-    ) + (chunker.chunks[0].summary_chars // 4)
+    assert done["payload"]["args"]["context_tokens"] < 75_000
 
 
 def test_leading_stampless_turn_joins_the_next_segment_instead_of_pinning_the_queue():
@@ -1238,13 +1249,14 @@ def test_leading_stampless_turn_joins_the_next_segment_instead_of_pinning_the_qu
     the head of the awaiting queue, so no L2 swap ever happened and the hard
     line jumped straight to the fold. It joins the following segment."""
     steps = [_step(i) for i in range(1, 13)]
-    meter = {"value": 50_000}  # above the start gate
+    meter = {"value": 40_000}  # above the start gate
     ledger, chunker, _, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     ledger.stage_turn([_observation(0), AIMessage(content="no tool call this turn")])
     _run_turns(ledger, chunker, 1, 7, _hashes(4))  # commits turn 0 without a step key
 
     assert [c.capsule_key for c in chunker.awaiting_chunks] == ["chunk:1-4"]
     assert all(entry["chunk"] is not None for entry in chunker._awaiting)
+    meter["value"] = 75_000
     capsule.resolve("chunk:1-4", _capsule(1, 4))
     _run_turns(ledger, chunker, 8, 8, _hashes(4))
 
@@ -1278,24 +1290,79 @@ def test_partition_folds_leading_unstamped_turns_into_the_first_segment():
     ]
 
 
-def test_start_gate_zero_restores_swap_on_ready():
+def test_start_gate_zero_only_prepares_and_does_not_rewrite_prefix():
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 9)]
     ledger, chunker, _, _ = _make(
         steps, min_active=2, meter=lambda: 1_000, auto_capsule=True, start_ratio=0.0
     )
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
-    assert len(chunker.chunks) == 1
-    assert chunker.awaiting_chunks == ()
+    assert chunker.chunks == ()
+    assert chunker.awaiting_chunks[0].status == "ready"
 
 
-def test_start_gate_unknown_base_falls_through_to_swap_on_ready():
-    """No measured operator call (meter None, nothing recorded on the ledger):
-    the gate cannot decide and the legacy swap-on-ready behavior stands."""
+def test_unknown_usage_uses_current_prompt_estimate_and_preserves_small_prefix():
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 9)]
     ledger, chunker, _, _ = _make(steps, min_active=2, auto_capsule=True)
-    assert ledger.last_prompt_tokens is None
+    chunker._meter_getter = None
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
-    assert len(chunker.chunks) == 1
+    assert ledger.last_prompt_tokens is None
+    assert ledger.projected_prompt_tokens > 0
+    assert chunker.chunks == ()
+    assert ledger.context_phase == 0
+
+
+def test_phase_batches_async_summaries_then_preserves_the_new_prefix():
+    steps = [_step(i) for i in range(1, 20)]
+    meter = {"value": 10_000}
+    ledger, chunker, _, capsule = _make(steps, meter=lambda: meter["value"])
+    ledger.set_static_prefix([HumanMessage(content="Stable rules")])
+    _run_turns(ledger, chunker, 1, 10, lambda i: "a" if i <= 4 else "b" if i <= 7 else "c")
+    assert capsule.submitted == []  # no background LLM work below preparation gate
+    tail = [_observation(11)]
+    original = ledger.render(tail)
+    fingerprint = [message.model_dump() for message in original]
+    meter["value"] = 40_000
+    ledger.render(tail)
+    assert len(chunker.awaiting_chunks) == 2
+    first, second = chunker.awaiting_chunks
+    capsule.resolve(first.capsule_key, _capsule(first.start_step_number, first.end_step_number))
+    meter["value"] = 75_000
+    assert [m.model_dump() for m in ledger.render(tail)] == fingerprint
+    assert ledger.context_phase == 0  # partial completion must not break the prefix
+    capsule.resolve(second.capsule_key, _capsule(second.start_step_number, second.end_step_number))
+    compacted = ledger.render(tail)
+    assert ledger.context_phase == 1
+    assert ledger.last_compaction["after_tokens"] < ledger.last_compaction["before_tokens"]
+    assert [m.model_dump() for m in original] == fingerprint  # prior input objects never mutated
+    assert len(chunker.chunks) == 2 and not chunker.awaiting_chunks
+
+    meter["value"] = 10_000  # provider confirms lower occupancy after compaction
+    # Commit the previous turn, then append a newly observed turn.
+    ledger.commit_staged(step_key="s10")
+    next_input = ledger.render(tail)
+    snapshot = [m.model_dump() for m in next_input]
+    turn = _turn(11)
+    turn[0] = tail[0]
+    ledger.stage_turn(turn)
+    ledger.commit_staged(step_key="s11")
+    following = ledger.render([_observation(12)])
+    assert [m.model_dump() for m in following[: len(snapshot)]] == snapshot
+    assert ledger.context_phase == 1
+    assert compacted[0].content == "Stable rules"
+
+
+def test_new_large_tail_triggers_pressure_without_waiting_for_next_usage():
+    steps = [_step(i) for i in range(1, 12)]
+    ledger, chunker, _, _ = _make(steps, auto_capsule=True)
+    chunker._meter_getter = None
+    previous = _run_turns(ledger, chunker, 1, 9, _hashes(4))
+    ledger.record_prompt_tokens(10_000, previous)
+    assert ledger.context_phase == 0
+    # New evidence already exceeds the hard line, despite a small last call.
+    ledger.render([HumanMessage(content="新证据" * 40_000)])
+    assert ledger.projected_prompt_tokens > 90_000
+    assert ledger.context_phase == 1
+    assert ledger.frozen_blocks
 
 
 def _big_turn(i: int, chars: int = 4000) -> list:
@@ -1323,7 +1390,7 @@ def test_hard_threshold_swaps_ready_prefix_as_l2_when_that_is_enough():
     estimated context back under the hard line, the swap is an ordinary L2
     event — no fold, and the younger pending segment stays awaiting."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 13)]
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, engine, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     # Steps 1–4 (hash-a) close at the switch; steps 5–8 (hash-b) close at the
     # next switch. Turns are ~1k tokens each so the L2 swap is worth ~4k.
@@ -1352,7 +1419,7 @@ def test_hard_threshold_escalates_to_fold_when_l2_swap_is_not_enough():
     and the whole frozen region folds into one recall-only era whose index
     lists every folded step — the pending chunk's steps too."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 13)]
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, engine, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 10, lambda i: "a" if i <= 4 else ("b" if i <= 8 else "c"))
     capsule.resolve("chunk:1-4", _capsule(1, 4))
@@ -1394,9 +1461,10 @@ def test_hard_fold_is_monotonic_later_l2_swaps_append_after_the_folded_era():
     flips back to full chunk blocks."""
     steps = [_step(i) for i in range(1, 17)]
     hash_for = lambda i: "a" if i <= 4 else ("b" if i <= 8 else ("c" if i <= 12 else "d"))  # noqa: E731
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, _, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 10, hash_for)
+    meter["value"] = 75_000
     capsule.resolve("chunk:1-4", _capsule(1, 4))
     meter["value"] = 99_000
     _run_turns(ledger, chunker, 11, 11, hash_for)  # hard: folds 1–4 and 5–8
@@ -1405,10 +1473,11 @@ def test_hard_fold_is_monotonic_later_l2_swaps_append_after_the_folded_era():
 
     # Pressure gone (above the start gate, below soft): the next milestone
     # switch closes 9–12; once ready it swaps as an ordinary L2 chunk.
-    meter["value"] = 50_000
+    meter["value"] = 40_000
     _run_turns(ledger, chunker, 12, 15, hash_for)
     pending = [c for c in chunker.awaiting_chunks if c.status == "pending"]
     assert [c.capsule_key for c in pending] == ["chunk:9-12"]
+    meter["value"] = 75_000
     capsule.resolve("chunk:9-12", _capsule(9, 12))
     _run_turns(ledger, chunker, 16, 16, hash_for)
 
@@ -1430,11 +1499,13 @@ def test_hard_threshold_with_nothing_awaiting_folds_frozen_region_once():
     into a recall-only era without consuming turns, and repeated hard renders
     leave the frozen blocks byte-identical."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 13)]
-    meter = {"value": 50_000}
+    meter = {"value": 40_000}
     ledger, chunker, _, _ = _make(
         steps, min_active=2, meter=lambda: meter["value"], auto_capsule=True
     )
-    _run_turns(ledger, chunker, 1, 7, _hashes(4))
+    _run_turns(ledger, chunker, 1, 6, _hashes(4))
+    meter["value"] = 75_000
+    _run_turns(ledger, chunker, 7, 7, _hashes(4))
     # Chunk 1–4 swapped as L2 at the step-7 render; s5..s6 remain = the floor.
     assert [c.capsule_key for c in chunker.chunks] == ["chunk:1-4"]
     assert chunker.awaiting_chunks == ()
@@ -1479,14 +1550,15 @@ def test_context_base_reads_the_ledger_operator_measurement_not_the_session_mete
     # Some other agent's small call lands on the session meter.
     get_meter("session-1").record({"prompt_tokens": 2_700, "completion_tokens": 10})
     # The operator's own measured prompt is well below the gate.
-    ledger.record_prompt_tokens(20_000)
-    assert chunker._context_base_tokens(ledger) == 20_000
+    chunker._meter_getter = None
+    ledger.record_prompt_tokens(40_000)
+    assert chunker._context_base_tokens(ledger) == 40_000
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
     assert chunker.chunks == ()  # held: 20k < 35k
     assert [c.status for c in chunker.awaiting_chunks] == ["ready"]
 
     # Operator prompt grows past the gate → swap at the next render.
-    ledger.record_prompt_tokens(36_000)
+    ledger.record_prompt_tokens(75_000)
     _run_turns(ledger, chunker, 9, 9, _hashes(4))
     assert len(chunker.chunks) == 1
 
@@ -1507,7 +1579,7 @@ def test_held_note_is_fresh_after_a_retry():
     then readied while the start gate still holds it gets a fresh ``held``
     note: the re-dispatch resets the once-per-chunk held marker."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 14)]
-    meter = {"value": 20_000}  # < 0.35 * 100k: the gate holds ready chunks
+    meter = {"value": 40_000}  # < 0.35 * 100k: the gate holds ready chunks
     ledger, chunker, engine, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
     entry = chunker._awaiting[0]
@@ -1797,7 +1869,7 @@ def test_calibrated_ratio_scales_the_size_trigger():
     # Default ratio: the eligible text (two ~4k-char turns once turn 5 is
     # staged) is worth ~chars/4 tokens, under the 5k target → nothing closes.
     ledger, chunker, _, _ = _make(
-        steps, min_active=2, min_steps=2, max_steps=100, target_tokens=5000
+        steps, min_active=2, min_steps=2, max_steps=100, target_tokens=5000, meter=lambda: 40_000
     )
     ledger.record_prompt_tokens(1000, messages=[HumanMessage(content="a" * 4000)])
     _run_big_turns(ledger, chunker, 1, 5, lambda i: "hash-a")
@@ -1807,7 +1879,7 @@ def test_calibrated_ratio_scales_the_size_trigger():
 
     # Calibrated to 1 char/token: the same text is worth ~chars tokens → closes.
     ledger, chunker, _, _ = _make(
-        steps, min_active=2, min_steps=2, max_steps=100, target_tokens=5000
+        steps, min_active=2, min_steps=2, max_steps=100, target_tokens=5000, meter=lambda: 40_000
     )
     ledger.record_prompt_tokens(4000, messages=[HumanMessage(content="中" * 4000)])
     assert ledger.chars_per_token == pytest.approx(1.0)
@@ -1840,6 +1912,7 @@ def test_compression_announces_one_tool_line_from_dispatch_to_swap():
     assert args["source_tokens"] > 0
     assert "context_tokens" not in args
 
+    meter["value"] = 75_000
     capsule.resolve("chunk:1-4", _capsule(1, 4))
     ledger.commit_staged(step_key="s8", validator_result={"status": "dispatched"})
     ledger.render([_observation(9)])
@@ -1853,16 +1926,14 @@ def test_compression_announces_one_tool_line_from_dispatch_to_swap():
     assert done_args["summary_tokens"] > 0
     assert done_args["context_budget"] == 100_000
     assert done_args["context_estimated"] is True
-    assert done_args["context_tokens"] == (
-        60_000 - done_args["source_tokens"] + done_args["summary_tokens"]
-    )
+    assert 0 < done_args["context_tokens"] < 75_000
     assert "forced" not in done_args
 
 
 def test_forced_swap_announces_fold_without_a_ratio():
     """Forced folds report estimated context usage without a summary size."""
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 9)]
-    meter = {"value": None}
+    meter = {"value": 40_000}
     ledger, chunker, engine, _ = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
     assert any(t["status"] == "running" for t in engine.traces)
@@ -1896,7 +1967,7 @@ def test_compression_trace_carries_a_plain_language_phase_on_every_path():
     assert COMPRESSION_PHASES == ("summarizing", "ready", "applied", "failed")
 
     steps = [_step(i, "hash-a" if i <= 4 else "hash-b") for i in range(1, 15)]
-    meter = {"value": 20_000}  # < 0.35 * 100k: the start gate holds ready chunks
+    meter = {"value": 40_000}  # < 0.35 * 100k: the start gate holds ready chunks
     ledger, chunker, engine, capsule = _make(steps, min_active=2, meter=lambda: meter["value"])
     _run_turns(ledger, chunker, 1, 8, _hashes(4))
     chunk = chunker.awaiting_chunks[0]
@@ -1933,9 +2004,14 @@ def test_compression_trace_carries_a_plain_language_phase_on_every_path():
     assert held["payload"]["args"]["note"] == "held"
 
     # Context past the gate → the swap applies the summary.
-    meter["value"] = 40_000
+    for pending in chunker.awaiting_chunks:
+        capsule.resolve(
+            pending.capsule_key, _capsule(pending.start_step_number, pending.end_step_number)
+        )
+    meter["value"] = 75_000
     _run_turns(ledger, chunker, 13, 13, lambda i: "hash-c")
-    assert [c.start_step_number for c in chunker.chunks] == [1]
+    assert chunker.chunks[0].start_step_number == 1
+    assert not chunker.awaiting_chunks
     assert mine()[-1] == ("success", "applied")
 
     # Every announcement on every trace carries a known phase.
diff --git a/tests/unit/memory/test_transcript_ledger.py b/tests/unit/memory/test_transcript_ledger.py
index db48ded..dcec926 100644
--- a/tests/unit/memory/test_transcript_ledger.py
+++ b/tests/unit/memory/test_transcript_ledger.py
@@ -71,9 +71,19 @@ def _turn(i: int, tool_call_count: int = 1) -> list:
     return messages
 
 
-def _play_turn(ledger: TranscriptLedger, i: int, *, prev_key=None, prev_result=None, **turn_kwargs):
+def _play_turn(
+    ledger: TranscriptLedger,
+    i: int,
+    *,
+    prev_key=None,
+    prev_result=None,
+    compact=False,
+    **turn_kwargs,
+):
     """Mimic the operator's real ordering: commit previous, render, stage."""
     ledger.commit_staged(step_key=prev_key, validator_result=prev_result)
+    if compact:
+        ledger.compact_active()
     turn = _turn(i, **turn_kwargs)
     rendered = ledger.render([turn[0]])
     ledger.stage_turn(turn)
@@ -159,8 +169,8 @@ def test_recitation_and_ui_list_stripped_at_the_text_edge_and_screenshot_at_k():
     until depth K and is then resolved in place; only the live tail ever
     carries an indexed element list."""
     ledger = TranscriptLedger(step_memory=_service(), image_scrub_depth=3)
-    _play_turn(ledger, 1)
-    rendered = _play_turn(ledger, 2, prev_key="step-1")
+    _play_turn(ledger, 1, compact=True)
+    rendered = _play_turn(ledger, 2, compact=True, prev_key="step-1")
     obs1 = ledger.active_messages[0]
     # Turn 1 is now depth 2: text edge passed, screenshot kept.
     assert PLAN_RECITATION_MARKER not in str(obs1.content)
@@ -168,10 +178,10 @@ def test_recitation_and_ui_list_stripped_at_the_text_edge_and_screenshot_at_k():
     assert _has_image(obs1)
     original_obs1 = _fingerprint(obs1)
 
-    rendered = _play_turn(ledger, 3, prev_key="step-2")
+    rendered = _play_turn(ledger, 3, compact=True, prev_key="step-2")
     # Turn 1 reached depth 3: the screenshot is resolved.
-    assert not _has_image(obs1)
-    assert _fingerprint(obs1) != original_obs1
+    assert not _has_image(ledger.active_messages[0])
+    assert _fingerprint(ledger.active_messages[0]) != original_obs1
     # Turn 2 (depth 2): text edge passed, screenshot still there; the live tail keeps both.
     obs2 = next(m for m in ledger.active_messages if "OBSERVATION [T+00:02]" in str(m.content))
     assert PLAN_RECITATION_MARKER not in str(obs2.content)
@@ -189,7 +199,7 @@ def test_depth_k_image_resolved_to_ready_visual_summary():
 
     ledger = TranscriptLedger(step_memory=service, image_scrub_depth=3)
     for i in range(1, 5):
-        _play_turn(ledger, i, prev_key=f"step-{i - 1}" if i > 1 else None)
+        _play_turn(ledger, i, compact=True, prev_key=f"step-{i - 1}" if i > 1 else None)
 
     first_obs = ledger.active_messages[0]
     assert not _has_image(first_obs)
@@ -212,7 +222,7 @@ def test_pending_grace_then_placeholder_never_backfilled():
     ledger = TranscriptLedger(step_memory=service, image_scrub_depth=2, pending_grace_steps=1)
     # Enough turns to push turn 1 past K + grace.
     for i in range(1, 7):
-        _play_turn(ledger, i, prev_key=f"step-{i - 1}" if i > 1 else None)
+        _play_turn(ledger, i, compact=True, prev_key=f"step-{i - 1}" if i > 1 else None)
 
     first_obs = ledger.active_messages[0]
     assert not _has_image(first_obs)
@@ -221,7 +231,7 @@ def test_pending_grace_then_placeholder_never_backfilled():
 
     # A late summary must never mutate the frozen message.
     service._summaries["step-1"] = "Too late."
-    _play_turn(ledger, 7, prev_key="step-6")
+    _play_turn(ledger, 7, compact=True, prev_key="step-6")
     assert _fingerprint(ledger.active_messages[0]) == frozen
 
 
@@ -232,7 +242,7 @@ def test_failed_summary_becomes_unavailable_placeholder():
 
     ledger = TranscriptLedger(step_memory=service, image_scrub_depth=2, pending_grace_steps=5)
     for i in range(1, 4):
-        _play_turn(ledger, i, prev_key=f"step-{i - 1}" if i > 1 else None)
+        _play_turn(ledger, i, compact=True, prev_key=f"step-{i - 1}" if i > 1 else None)
 
     first_obs = ledger.active_messages[0]
     assert not _has_image(first_obs)
@@ -473,7 +483,7 @@ def test_relaxed_depth_below_start_gate_keeps_more_screenshots():
     assert ledger.occupancy is None
     assert ledger.effective_image_scrub_depth == 6
     for i in range(1, 10):
-        _play_turn(ledger, i, prev_key=f"step-{i - 1}" if i > 1 else None)
+        _play_turn(ledger, i, compact=True, prev_key=f"step-{i - 1}" if i > 1 else None)
     # At the render of turn 9 its observation is the live tail (depth 1);
     # committed turns 8..5 are depths 2..5 and turn 4 (depth 6) is scrubbed.
     ledger.record_prompt_tokens(20_000)  # 20% < 35%
@@ -482,34 +492,34 @@ def test_relaxed_depth_below_start_gate_keeps_more_screenshots():
 
     tight = TranscriptLedger(step_memory=_service(), image_scrub_depth=3)
     for i in range(1, 10):
-        _play_turn(tight, i, prev_key=f"step-{i - 1}" if i > 1 else None)
+        _play_turn(tight, i, compact=True, prev_key=f"step-{i - 1}" if i > 1 else None)
     assert _image_turns(tight) == [8]
 
 
 def test_crossing_the_start_gate_tightens_once_and_relaxing_never_backfills():
     ledger = _banded_ledger()
     for i in range(1, 10):
-        _play_turn(ledger, i, prev_key=f"step-{i - 1}" if i > 1 else None)
+        _play_turn(ledger, i, compact=True, prev_key=f"step-{i - 1}" if i > 1 else None)
     assert _image_turns(ledger) == [5, 6, 7, 8]
 
     # Past the gate: the next render scrubs the extra depths in one pass
     # (turns 5–8 were depths 3–6 at that render; only turn 9 at depth 2 stays).
     ledger.record_prompt_tokens(40_000)  # 40% >= 35%
     assert ledger.effective_image_scrub_depth == 3
-    _play_turn(ledger, 10, prev_key="step-9")
+    _play_turn(ledger, 10, compact=True, prev_key="step-9")
     assert _image_turns(ledger) == [9]
 
     # Back under the gate: nothing is restored; new turns accumulate again
     # until four historical screenshots are in view.
     ledger.record_prompt_tokens(20_000)
     assert ledger.effective_image_scrub_depth == 6
-    _play_turn(ledger, 11, prev_key="step-10")
+    _play_turn(ledger, 11, compact=True, prev_key="step-10")
     assert _image_turns(ledger) == [9, 10]
-    _play_turn(ledger, 12, prev_key="step-11")
+    _play_turn(ledger, 12, compact=True, prev_key="step-11")
     assert _image_turns(ledger) == [9, 10, 11]
-    _play_turn(ledger, 13, prev_key="step-12")
+    _play_turn(ledger, 13, compact=True, prev_key="step-12")
     assert _image_turns(ledger) == [9, 10, 11, 12]
-    _play_turn(ledger, 14, prev_key="step-13")  # turn 9 reaches depth 6
+    _play_turn(ledger, 14, compact=True, prev_key="step-13")  # turn 9 reaches depth 6
     assert _image_turns(ledger) == [10, 11, 12, 13]
 
 
@@ -523,10 +533,8 @@ def test_no_band_configuration_keeps_the_fixed_depth():
     assert relaxed_only.effective_image_scrub_depth == 3  # no gate → never relaxed
 
 
-def test_scrub_edge_runs_before_the_chunker_at_render():
-    """The chunker sees the active window after the scrub edge advanced, so a
-    chunk closing over turns older than the floor never captures a live
-    screenshot in its source transcript."""
+def test_chunker_sees_immutable_raw_history_before_a_phase_boundary():
+    """Preparing capsules must not scrub any previously sent observation."""
     ledger = _banded_ledger()
     seen: list[list[int]] = []
 
@@ -537,9 +545,9 @@ def test_scrub_edge_runs_before_the_chunker_at_render():
     ledger.attach_chunker(Probe())
     for i in range(1, 9):
         _play_turn(ledger, i, prev_key=f"step-{i - 1}" if i > 1 else None)
-    # At the render of turn 8 the scrub edge (depth 6) had already resolved
-    # turn 3 (depth 6); the probe saw turns 4–7 with images, never turn 3.
-    assert seen[-1] == [4, 5, 6, 7]
+    # A render below pressure preserves every historical image.
+    assert seen[-1] == list(range(1, 8))
+    assert ledger.context_phase == 0
 
 
 # ---------------------------------------------------------------------------
@@ -580,6 +588,7 @@ def test_ephemeral_blocks_vanish_at_the_text_edge_and_not_before():
     ledger.stage_turn(turn_with_notice(1))
     for i in range(2, 4):
         ledger.commit_staged(step_key=f"step-{i - 1}")
+        ledger.compact_active()
         ledger.render([_observation(i)])
         ledger.stage_turn(turn_with_notice(i))
 
@@ -600,15 +609,12 @@ def test_ephemeral_blocks_vanish_at_the_text_edge_and_not_before():
 
 
 def test_rendering_past_the_edge_is_byte_identical_and_prefix_stable():
-    """(iii) A render with nothing new is byte-identical to the previous one,
-    and across turns exactly one message — the observation reaching depth K —
-    differs from the previous render's prefix, so the request prefix before
-    it is what the provider's prompt cache already holds."""
+    """Every full previous prompt, including its live tail, survives unchanged."""
     ledger = TranscriptLedger(step_memory=_ready_service(12), image_scrub_depth=3)
     ledger.set_static_prefix([SystemMessage(content="STATIC")])
 
     def snapshot(rendered: list) -> list[str]:
-        return [_fingerprint(m) for m in rendered[:-1]]  # everything but the live tail
+        return [_fingerprint(m) for m in rendered]  # includes the previously live observation
 
     previous: list[str] = []
     for i in range(1, 11):
@@ -620,24 +626,15 @@ def test_rendering_past_the_edge_is_byte_identical_and_prefix_stable():
         )
         current = snapshot(rendered)
         changed = [idx for idx, fp in enumerate(previous) if current[idx] != fp]
-        if i >= 3:
-            # The live tail is depth 1 and committed turn i-1 depth 2, so the
-            # turn reaching depth 3 is i-2; its observation heads its span.
-            expected = next(
-                idx
-                for idx, m in enumerate(rendered[:-1])
-                if f"OBSERVATION [T+00:0{(i - 2) % 10}]" in str(m.content)
-            )
-            assert changed == [expected], (i, changed)
-        else:
-            assert changed == [], (i, changed)
+        assert changed == [], (i, changed)
         previous = current
 
     # Same state, rendered again: byte-identical, including the tail.
+    ledger.commit_staged(step_key="step-10", validator_result={"status": "dispatched"})
     tail = [_observation(11)]
     again = [_fingerprint(m) for m in ledger.render(tail)]
     assert again == [_fingerprint(m) for m in ledger.render(tail)]
-    assert again[:-1] == previous
+    assert again[: len(previous)] == previous
 
 
 # ---------------------------------------------------------------------------
diff --git a/tests/unit/test_memory_config.py b/tests/unit/test_memory_config.py
index a4b9fe2..3be6a0f 100644
--- a/tests/unit/test_memory_config.py
+++ b/tests/unit/test_memory_config.py
@@ -134,6 +134,8 @@ def test_xml_scrub_depth_positions_the_text_edge():
     ledger.stage_turn([obs(1)])
     ledger.commit_staged(step_key="s1")
     ledger.render([obs(2)])
+    assert PRO_UI_LIST_MARKER in str(ledger.active_messages[0].content)
+    ledger.compact_active()  # depth settings apply at a compaction boundary
     committed = ledger.active_messages[0]
     assert PRO_UI_LIST_MARKER not in str(committed.content)
     assert any(isinstance(b, dict) and b.get("type") == "image_url" for b in committed.content)
diff --git a/tests/unit/test_plugin_result_delivery.py b/tests/unit/test_plugin_result_delivery.py
index 312d079..0a057d8 100644
--- a/tests/unit/test_plugin_result_delivery.py
+++ b/tests/unit/test_plugin_result_delivery.py
@@ -65,7 +65,9 @@ def test_burst_results_reach_responses_once_with_images_before_compression(tmp_p
     actions[2].pop("post_image_name")
     actions[2]["attempts"] = ["Skipped"]
     report = {"status": "failed", "execution": actions}
-    ledger = TranscriptLedger(image_scrub_depth=1, pending_grace_steps=0)
+    ledger = TranscriptLedger(
+        image_scrub_depth=1, pending_grace_steps=0, context_budget_tokens=300_000
+    )
     _stage(ledger, actions)
     ledger.commit_staged(
         step_key="step1",
@@ -84,13 +86,17 @@ def test_burst_results_reach_responses_once_with_images_before_compression(tmp_p
     assert len([item for item in payload if item.get("type") == "function_call_output"]) == 4
     assert str(payload).count(base64.b64encode(image.read_bytes()).decode()) == 3
     assert f"data:{Image.MIME[image_format]};base64," in str(payload)
-    # No duplicate outputs at the following turn; old images are eligible for scrubbing again.
+    # No duplicate outputs or rewritten images on the following ordinary turn.
     ledger.commit_staged(
         validator_result=report, tool_results=plugin_result_messages(report, engine)
     )
     later = ledger.render([])
     assert len([m for m in later if isinstance(m, ToolMessage)]) == 4
+    assert str(later).count(base64.b64encode(image.read_bytes()).decode()) == 3
+    ledger.compact_active()
+    later = ledger.render([])
     assert str(later).count(base64.b64encode(image.read_bytes()).decode()) < 3
+    assert str(messages).count(base64.b64encode(image.read_bytes()).decode()) == 3
 
 
 def test_missing_plugin_screenshot_keeps_real_result_and_reports_missing_evidence(tmp_path):
diff --git a/tests/unit/test_prefix_cache.py b/tests/unit/test_prefix_cache.py
new file mode 100644
index 0000000..0640380
--- /dev/null
+++ b/tests/unit/test_prefix_cache.py
@@ -0,0 +1,63 @@
+import json
+from types import SimpleNamespace
+
+from langchain_core.messages import AIMessage, HumanMessage, SystemMessage, ToolMessage
+
+from artemis.llm.prefix_cache import PromptPrefixTracker
+from artemis.llm.router import ModelEndpoint
+from artemis.services.llm import RobustChatModelWrapper
+
+
+def test_prefix_diagnostics_distinguish_append_rewrite_and_tool_changes_without_payloads():
+    tracker = PromptPrefixTracker()
+    messages = [SystemMessage(content="private rules"), HumanMessage(content="private screenshot")]
+    options = {"tools": [{"name": "click"}, {"name": "observe"}]}
+    assert tracker.observe("task:flash", messages, options)["cold_start"]
+    messages.extend(
+        [
+            AIMessage(content="", tool_calls=[{"name": "click", "args": {"x": 5}, "id": "c1"}]),
+            ToolMessage(content="clicked", tool_call_id="c1"),
+        ]
+    )
+    extended = tracker.observe("task:flash", messages, options)
+    assert extended["append_only"] and extended["shared_prefix_messages"] == 2
+    messages[1] = HumanMessage(content="compressed summary")
+    rewritten = tracker.observe("task:flash", messages, options)
+    assert rewritten["first_changed_message"] == 2 and not rewritten["append_only"]
+    reordered = tracker.observe("task:flash", messages, {"tools": list(reversed(options["tools"]))})
+    assert reordered["tools_or_format_changed"] and not reordered["append_only"]
+    assert tracker.observe("task:checker", messages, options)["cold_start"]
+    assert "private" not in json.dumps(tracker._previous)
+
+
+def test_bound_wrappers_reuse_task_prefix_tracker_after_message_adaptation():
+    class Model:
+        kwargs = {"tools": [{"type": "function", "name": "observe"}]}
+
+        def bind_tools(self, *_args, **_kwargs):
+            return self
+
+    ctx = SimpleNamespace(data_engine=SimpleNamespace(current_session_id="session"))
+    wrapper = RobustChatModelWrapper(Model(), ctx=ctx, endpoint=ModelEndpoint(), role="flash")
+    messages = [SystemMessage(content="rules"), HumanMessage(content="step 1")]
+
+    def diagnostics(items):
+        bound = wrapper.bind_tools([])  # Flash binds again for each call
+        args, kwargs = bound._prepare_model_call((items,), {})
+        return bound._diagnostics(args, kwargs)
+
+    first = diagnostics(messages)
+    messages.append(HumanMessage(content="step 2"))
+    second = diagnostics(messages)
+    assert first["prefix_cache"]["cold_start"]
+    assert second["prefix_cache"]["append_only"]
+    assert first["prompt_cache_key"] == second["prompt_cache_key"]
+    assert not any("rules" in str(value) for value in ctx.prompt_prefix_tracker._previous.values())
+
+    ctx.transcript_ledger = SimpleNamespace(
+        context_phase=1, last_compaction={"reason": "context_pressure"}
+    )
+    third = diagnostics(messages[:1] + [HumanMessage(content="history summary")])
+    assert third["prefix_cache"]["context_phase"] == 1
+    assert third["prefix_cache"]["last_compaction"]["reason"] == "context_pressure"
+    assert not third["prefix_cache"]["append_only"]