diff --git a/backend/AGENTS.md b/backend/AGENTS.md
index 0b12af195..9bad4862a 100644
--- a/backend/AGENTS.md
+++ b/backend/AGENTS.md
@@ -379,8 +379,9 @@ Proxied through nginx: `/api/langgraph/*` → Gateway LangGraph-compatible runti
 **Concurrency**: `MAX_CONCURRENT_SUBAGENTS = 3` enforced by `SubagentLimitMiddleware` (truncates excess tool calls in `after_model`); default subagent timeout `subagents.timeout_seconds=1800` (30 min) and built-in `general-purpose` `max_turns=150` (raised from 100/15-min so deep-research subtasks stop hitting `GraphRecursionError` out of the box)
 **Flow**: `task()` tool → `SubagentExecutor` → background thread → poll 5s → SSE events → result
 **Events**: `task_started`, `task_running`, `task_completed`/`task_failed`/`task_timed_out`
-**Guardrail caps & `stop_reason` (#3875 Phase 2)**: three independent axes can end a subagent run early, and all now surface *why* through one additive field rather than a new status enum. **Turn axis**: `recursion_limit` on the subagent `run_config` equals `max_turns`, so exhausting the turn budget raises `GraphRecursionError` from `agent.astream`; `executor.py::_aexecute` catches it specifically (before the generic `except Exception`). **Token axis**: `TokenBudgetMiddleware` is attached per-agent via `build_subagent_runtime_middlewares` from `subagents.token_budget` (default 2,000,000 tokens, warn at 0.7, hard-stop at 1.0 — a backstop against a subagent that burns tokens on trivial work). It does *not* raise: at the hard-stop threshold it strips the in-flight turn's tool calls, forces `finish_reason="stop"`, and lets the run complete naturally with a final answer. **Loop axis**: `LoopDetectionMiddleware` (attached at the same point) catches repeated identical tool-call sets — or one tool *type* called many times with varying args — and its hard-stop likewise strips `tool_calls` and forces a final answer without raising, recording `loop_capped`. Each guard exposes its cap on a per-`run_id` `consume_stop_reason(run_id)` accessor; `_aexecute` collects **every** middleware with that method (duck-typed via `hasattr`, so the executor has no import coupling to the guard classes) and surfaces the first non-`None` reason — adding a future guard needs no executor change. **Surfacing**: whichever axis fired, `_aexecute` stamps a normal status plus an additive reason — `completed` + `stop_reason=token_capped|turn_capped|loop_capped` when a usable final answer (or partial recovered from the last streamed chunk via `_extract_final_result` → `utils/messages.py::message_content_to_text`, returning a `"No response Generated"` sentinel when no text survived) was produced; `failed` + `stop_reason=turn_capped` when nothing usable survived. `SubagentResult.stop_reason` flows through `task_tool.py::_task_result_command` → `format_subagent_result_message` (renders `Task Succeeded (capped: ...)` / `Task failed (capped: ...)`) and `make_subagent_additional_kwargs`, which stamps the additive `subagent_stop_reason` key alongside the normal `subagent_status`. **Why additive, not an enum**: a new status value would break v1 consumers; an optional field is ignored by older frontends and ledger readers, so the cross-language contract (`contracts/subagent_status_contract.json` v2 + `subagents/status_contract.py` + `frontend/.../subtask-result.ts`, pinned by `test_status_values_match_contract` / `test_stop_reason_values_match_contract`) stays backward-compatible. The durable delegation ledger captures `stop_reason` onto the entry and renders model-facing guidance ("hit a guardrail cap with a partial result; reuse it, retry tighter, or raise the per-agent budget (`max_turns` / `token_budget`)") so the lead reuses a capped completion knowingly instead of mistaking it for a clean one. (Phase 1 shipped this surfacing as a `MAX_TURNS_REACHED` status enum in #3949; Phase 2 replaced that enum with the additive `stop_reason` field per the agreed design — the `max_turns_reached` status value and `SubagentStatus.MAX_TURNS_REACHED` are gone.)
-**Step capture & persistence (#3779)**: `executor.py` captures both assistant turns (`AIMessage`) **and** tool outputs (`ToolMessage`) via `subagents/step_events.py::capture_new_step_messages`, which walks the *newly-appended tail* of each `stream_mode="values"` chunk (not just `messages[-1]`) so a multi-tool-call turn — where LangGraph's `ToolNode` appends several `ToolMessage`s in one super-step — keeps every tool output instead of dropping all but the last. `runtime/runs/worker.py::_SubagentEventBuffer` additionally persists these `task_*` custom events to the `RunEventStore` as `subagent.start`/`subagent.step`/`subagent.end` (`category="subagent"`, `task_id` in `metadata`). It **batches** writes via `put_batch` (flushing on a terminal `subagent.end`, at `FLUSH_THRESHOLD` events, and in the worker's `finally`) rather than one `put()` per step, since `put()` is a documented low-frequency path (per-thread advisory lock per call) and a deep subagent (`max_turns=150`) emits hundreds of steps on the hot stream loop. `build_subagent_step` caps both the per-step `text` and each tool call's serialized `args` at `SUBAGENT_STEP_MAX_CHARS` (flagged `truncated` / `args_truncated`) so a large `write_file`/`bash` payload can't produce an unbounded row. The dedicated category keeps them out of `list_messages` (the thread feed) while `list_events` returns them for the frontend's fetch-on-expand backfill. `list_events` accepts `task_id` (filters on `metadata["task_id"]` — SQL-side in `DbRunEventStore` via `event_metadata["task_id"].as_string()`, in-memory in the JSONL/memory stores) plus an `after_seq` forward cursor, so the card pages through one subagent's steps without the run-wide `limit` truncating the tail (no schema migration: the filter rides the existing run-scoped index). `step_events.py` is a pure, unit-tested layer (`build_subagent_step` / `subagent_run_event`).
+**Guardrail caps & `stop_reason` (#3875 Phase 2)**: three independent axes can end a subagent run early, and all now surface *why* through one additive field rather than a new status enum. **Turn axis**: `recursion_limit` on the subagent `run_config` equals `max_turns`, so exhausting the turn budget raises `GraphRecursionError` from `agent.astream`; `executor.py::_aexecute` catches it specifically (before the generic `except Exception`). **Token axis**: `TokenBudgetMiddleware` is attached per-agent via `build_subagent_runtime_middlewares` from `subagents.token_budget` (default `max_tokens` **coupled to `summarization.enabled`** — 1,000,000 when subagent summarization is on, 2,000,000 when off, warn at 0.7, hard-stop at 1.0; a user-set budget always wins regardless of the switch — #3875 Phase 3; a backstop against a subagent that burns tokens on trivial work). It does *not* raise: at the hard-stop threshold it strips the in-flight turn's tool calls, forces `finish_reason="stop"`, and lets the run complete naturally with a final answer. **Loop axis**: `LoopDetectionMiddleware` (attached at the same point) catches repeated identical tool-call sets — or one tool *type* called many times with varying args — and its hard-stop likewise strips `tool_calls` and forces a final answer without raising, recording `loop_capped`. Each guard exposes its cap on a per-`run_id` `consume_stop_reason(run_id)` accessor; `_aexecute` collects **every** middleware with that method (duck-typed via `hasattr`, so the executor has no import coupling to the guard classes) and surfaces the first non-`None` reason — adding a future guard needs no executor change. **Surfacing**: whichever axis fired, `_aexecute` stamps a normal status plus an additive reason — `completed` + `stop_reason=token_capped|turn_capped|loop_capped` when a usable final answer (or partial recovered from the last streamed chunk via `_extract_final_result` → `utils/messages.py::message_content_to_text`, returning a `"No response Generated"` sentinel when no text survived) was produced; `failed` + `stop_reason=turn_capped` when nothing usable survived. `SubagentResult.stop_reason` flows through `task_tool.py::_task_result_command` → `format_subagent_result_message` (renders `Task Succeeded (capped: ...)` / `Task failed (capped: ...)`) and `make_subagent_additional_kwargs`, which stamps the additive `subagent_stop_reason` key alongside the normal `subagent_status`. **Why additive, not an enum**: a new status value would break v1 consumers; an optional field is ignored by older frontends and ledger readers, so the cross-language contract (`contracts/subagent_status_contract.json` v2 + `subagents/status_contract.py` + `frontend/.../subtask-result.ts`, pinned by `test_status_values_match_contract` / `test_stop_reason_values_match_contract`) stays backward-compatible. The durable delegation ledger captures `stop_reason` onto the entry and renders model-facing guidance ("hit a guardrail cap with a partial result; reuse it, retry tighter, or raise the per-agent budget (`max_turns` / `token_budget`)") so the lead reuses a capped completion knowingly instead of mistaking it for a clean one. (Phase 1 shipped this surfacing as a `MAX_TURNS_REACHED` status enum in #3949; Phase 2 replaced that enum with the additive `stop_reason` field per the agreed design — the `max_turns_reached` status value and `SubagentStatus.MAX_TURNS_REACHED` are gone.)
+**Context compaction (#3875 Phase 3)**: subagents inherit `DeerFlowSummarizationMiddleware` via `build_subagent_runtime_middlewares`, gated on the **same** `summarization.enabled` switch the lead reads (one config covers both chains; trigger/keep/model/prompt come from the shared `summarization` config so they cannot drift). The factory is called with `skip_memory_flush=True` on the subagent path: the lead's `memory_flush_hook` (attached when `memory.enabled`) flushes pre-compaction messages into durable memory keyed by `thread_id`, and subagents share the parent's `thread_id`, so without skipping the hook a subagent's internal turns would pollute the **parent** thread's durable memory. Placement differs from the lead chain (lead appends summarization *before* the guard trio; subagent appends it *after*) — benign because the middleware implements only `before_model` (compaction) with no `after_model`/`consume_stop_reason`, so it cannot disturb the Phase 2 guard-cap stop-reason channel. Compaction rewrites the messages channel via `RemoveMessage(id=REMOVE_ALL_MESSAGES)`, which shrinks `len(messages)` below the step-capture cursor mid-run; `capture_new_step_messages` (see Step capture below) resets the cursor to the new tail on contraction so steps appended after the compaction point are not silently dropped.
+**Step capture & persistence (#3779)**: `executor.py` captures both assistant turns (`AIMessage`) **and** tool outputs (`ToolMessage`) via `subagents/step_events.py::capture_new_step_messages`, which walks the *newly-appended tail* of each `stream_mode="values"` chunk (not just `messages[-1]`) so a multi-tool-call turn — where LangGraph's `ToolNode` appends several `ToolMessage`s in one super-step — keeps every tool output instead of dropping all but the last. `runtime/runs/worker.py::_SubagentEventBuffer` additionally persists these `task_*` custom events to the `RunEventStore` as `subagent.start`/`subagent.step`/`subagent.end` (`category="subagent"`, `task_id` in `metadata`). It **batches** writes via `put_batch` (flushing on a terminal `subagent.end`, at `FLUSH_THRESHOLD` events, and in the worker's `finally`) rather than one `put()` per step, since `put()` is a documented low-frequency path (per-thread advisory lock per call) and a deep subagent (`max_turns=150`) emits hundreds of steps on the hot stream loop. `build_subagent_step` caps both the per-step `text` and each tool call's serialized `args` at `SUBAGENT_STEP_MAX_CHARS` (flagged `truncated` / `args_truncated`) so a large `write_file`/`bash` payload can't produce an unbounded row. The dedicated category keeps them out of `list_messages` (the thread feed) while `list_events` returns them for the frontend's fetch-on-expand backfill. `list_events` accepts `task_id` (filters on `metadata["task_id"]` — SQL-side in `DbRunEventStore` via `event_metadata["task_id"].as_string()`, in-memory in the JSONL/memory stores) plus an `after_seq` forward cursor, so the card pages through one subagent's steps without the run-wide `limit` truncating the tail (no schema migration: the filter rides the existing run-scoped index). `step_events.py` is a pure, unit-tested layer (`build_subagent_step` / `subagent_run_event`). **History contraction (#3875 Phase 3)**: `capture_new_step_messages` assumes append-only growth, but `DeerFlowSummarizationMiddleware` rewrites the messages channel via `RemoveMessage(id=REMOVE_ALL_MESSAGES)`, shrinking `len(messages)` below the cursor mid-run. On contraction (`total < processed_count`) the cursor resets to the new tail; `capture_step_message`'s id/content dedup prevents re-emitting pre-compaction steps, so steps appended after the compaction point are still captured instead of being dropped until `total` overtakes the stale cursor.
 **Deferred MCP tools** (if `tool_search.enabled`): `SubagentExecutor._build_initial_state` assembles deferral after policy filtering via the shared `assemble_deferred_tools` (fail-closed), appends the `tool_search` tool, injects the `<available-deferred-tools>` section into the subagent's `SystemMessage`, and threads the setup to `_create_agent`, which attaches `McpRoutingMiddleware` (when PR1 routing metadata matches deferred tools) before `DeferredToolFilterMiddleware` through `build_subagent_runtime_middlewares(...)`. Subagents thus withhold full MCP schemas until promotion, same as the lead agent; each task run gets a fresh `ThreadState` so promotion is isolated per run
 **Checkpointer isolation**: Subagent graphs are compiled with `checkpointer=False` to avoid inheriting the parent run's checkpointer, since subagents are one-shot and never resume.
 
diff --git a/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py
index 59ac576a7..02c329fe3 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/summarization_middleware.py
@@ -437,15 +437,22 @@ def create_summarization_middleware(
     *,
     app_config: Any | None = None,
     keep: tuple[str, int | float] | None = None,
+    skip_memory_flush: bool = False,
 ) -> DeerFlowSummarizationMiddleware | None:
     """Create the configured summarization middleware.
 
     Both the lead-agent automatic path and the manual context-compaction path
     use this factory so model resolution, hooks, prompt config, and retention
     defaults cannot drift.
-    """
-    from deerflow.agents.memory.summarization_hook import memory_flush_hook
 
+    ``skip_memory_flush`` omits the ``memory_flush_hook`` that otherwise
+    flushes pre-compaction messages into the durable memory queue. The lead
+    chain keeps it (research should persist); the subagent chain sets it so a
+    subagent's INTERNAL turns (the "Task" human message + intermediate AI/tool
+    turns) are not written into the PARENT thread's durable memory — the hook
+    is keyed by ``thread_id`` and subagents share the parent's ``thread_id``
+    (#3875 Phase 3 review).
+    """
     resolved_app_config = app_config or get_app_config()
     config = resolved_app_config.summarization
 
@@ -485,7 +492,9 @@ def create_summarization_middleware(
         kwargs["summary_prompt"] = config.summary_prompt
 
     hooks: list[BeforeSummarizationHook] = []
-    if resolved_app_config.memory.enabled:
+    if resolved_app_config.memory.enabled and not skip_memory_flush:
+        from deerflow.agents.memory.summarization_hook import memory_flush_hook
+
         hooks.append(memory_flush_hook)
 
     return DeerFlowSummarizationMiddleware(
diff --git a/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py
index 03c7c9d6f..38ba66e78 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/tool_error_handling_middleware.py
@@ -353,7 +353,17 @@ def build_subagent_runtime_middlewares(
     # builds a fresh middleware instance (see ``executor._create_agent``), so
     # parallel subagents cannot cross-contaminate even though they share the
     # parent thread_id/run_id in context.
-    token_budget_config = app_config.subagents.get_token_budget_for(agent_name) if agent_name is not None else app_config.subagents.token_budget
+    #
+    # Default-ceiling coupling (#3875 Phase 3 review): the default ``max_tokens``
+    # is re-coupled to ``summarization.enabled`` — 1M when compaction is on, 2M
+    # when off. This ONLY applies to the default; a user-set budget (global or
+    # per-agent) always wins, so a deployment that pinned a value is never
+    # silently changed by flipping the summarization switch.
+    summarization_enabled = app_config.summarization.enabled
+    if agent_name is not None:
+        token_budget_config = app_config.subagents.get_token_budget_for(agent_name, summarization_enabled=summarization_enabled)
+    else:
+        token_budget_config = app_config.subagents.token_budget
     if token_budget_config.enabled:
         from deerflow.agents.middlewares.token_budget_middleware import TokenBudgetMiddleware
 
@@ -369,4 +379,45 @@ def build_subagent_runtime_middlewares(
 
         middlewares.append(SafetyFinishReasonMiddleware.from_config(safety_config))
 
+    # DeerFlowSummarizationMiddleware — subagents inherit none of the lead's
+    # context compaction today (#3875 Phase 3): a deep-research subagent
+    # (``max_turns`` up to 150) can accumulate >1M cumulative input before
+    # max_turns/timeout/token_budget engage, even though Phase 2's budget now
+    # caps the pathological tail. Gated on the SAME
+    # ``app_config.summarization.enabled`` switch the lead reads (per
+    # maintainer guidance in #3875) so a single config covers both chains —
+    # no separate ``subagents.summarization`` field. The shared factory
+    # returns ``None`` when summarization is disabled, so this is a pure
+    # no-op when the switch is off. Trigger/keep/model/prompt all come from
+    # the same ``summarization`` config the lead reads, so the two chains
+    # cannot drift.
+    #
+    # Placement differs from the lead chain: the lead appends summarization
+    # BEFORE the guard trio (loop/token/safety), here it is appended AFTER.
+    # This is benign — compaction runs in ``before_model`` regardless of
+    # relative position, and the guard middlewares account in ``after_model``
+    # — but noted because the relative order is not an exact mirror.
+    #
+    # ``skip_memory_flush=True``: the factory otherwise attaches
+    # ``memory_flush_hook`` (when ``memory.enabled``), which flushes
+    # pre-compaction messages into the durable memory queue keyed by
+    # ``thread_id``. Subagents share the parent's ``thread_id`` in context, so
+    # without skipping the hook a subagent's internal turns would be written
+    # into the PARENT thread's durable memory (#3875 Phase 3 review).
+    #
+    # The middleware rewrites history via ``RemoveMessage(id=REMOVE_ALL_MESSAGES)``,
+    # which shrinks the messages channel mid-run;
+    # ``capture_new_step_messages`` must tolerate that contraction (see
+    # ``step_events.py``) or it drops steps captured after the compaction
+    # point. It does not implement ``consume_stop_reason``, so it does not
+    # interfere with the Phase 2 guard-cap stop-reason channel.
+    from deerflow.agents.middlewares.summarization_middleware import create_summarization_middleware
+
+    summarization_middleware = create_summarization_middleware(
+        app_config=app_config,
+        skip_memory_flush=True,
+    )
+    if summarization_middleware is not None:
+        middlewares.append(summarization_middleware)
+
     return middlewares
diff --git a/backend/packages/harness/deerflow/config/subagents_config.py b/backend/packages/harness/deerflow/config/subagents_config.py
index 51e479f4a..7106a2602 100644
--- a/backend/packages/harness/deerflow/config/subagents_config.py
+++ b/backend/packages/harness/deerflow/config/subagents_config.py
@@ -9,18 +9,34 @@ from deerflow.config.token_budget_config import TokenBudgetConfig
 logger = logging.getLogger(__name__)
 
 
-def default_subagent_token_budget() -> TokenBudgetConfig:
-    """Default per-run token budget for subagents (#3875 Phase 2).
+def default_subagent_token_budget(*, summarization_enabled: bool = False) -> TokenBudgetConfig:
+    """Default per-run token budget for subagents (#3875 Phase 2 → Phase 3 coupling).
 
     Enabled by default so the pathological-token-burn backstop actually
     engages (per umbrella #3857 point 4 — backstops must engage, not just
-    exist). ``max_tokens`` is a deliberately loose ceiling: the reported 4.4M
-    burn would have been cut roughly in half, while legitimate deep-research
-    runs (``max_turns=150``, no summarization yet) can genuinely accumulate
-    >1M cumulative input today. Tighten after Phase 3 lands subagent
-    summarization. Flagged tunable in the PR description.
+    exist). ``max_tokens`` is **coupled to whether subagent summarization is
+    on** (#3875 Phase 3 review point):
+
+    - ``summarization_enabled=True`` (Phase 3 compacts the running context
+      before it reaches pathological size): **1M** — tighter ceiling still
+      covers legitimate deep research while catching degenerate runs earlier.
+    - ``summarization_enabled=False``: **2M** — the Phase 2 ceiling. Phase 2's
+      own docstring noted legitimate deep-research runs (``max_turns=150``,
+      no summarization) "can genuinely accumulate >1M cumulative input," so a
+      1M ceiling without compaction would prematurely cap them. Keeping 2M
+      here preserves that headroom; the tighter 1M only applies when the
+      compaction that justifies it is actually running.
+
+    The model-level ``default_factory`` (``SubagentsAppConfig.token_budget``)
+    cannot read ``summarization.enabled`` (a sibling top-level field), so it
+    falls back to the 2M no-compaction default; the builder
+    (``build_subagent_runtime_middlewares``) recomputes via
+    ``get_token_budget_for(..., summarization_enabled=...)`` so the live value
+    reflects the actual switch. A user-set ``token_budget`` (global or
+    per-agent) always wins regardless of the switch. Flagged tunable.
     """
-    return TokenBudgetConfig(enabled=True, max_tokens=2_000_000, warn_threshold=0.7)
+    max_tokens = 1_000_000 if summarization_enabled else 2_000_000
+    return TokenBudgetConfig(enabled=True, max_tokens=max_tokens, warn_threshold=0.7)
 
 
 class SubagentOverrideConfig(BaseModel):
@@ -114,6 +130,20 @@ class SubagentsAppConfig(BaseModel):
         description="User-defined subagent types keyed by agent name",
     )
 
+    # True when ``token_budget`` was NOT explicitly provided by the user, i.e.
+    # the field fell back to its default_factory. ``get_token_budget_for`` uses
+    # this to decide whether the ceiling may be re-coupled to
+    # ``summarization.enabled`` (#3875 Phase 3): a user-set budget is always
+    # respected as-is. Set by ``__init__`` from ``model_fields_set`` and
+    # preserved across the app-config reload path (which drops a default
+    # ``token_budget`` before re-constructing — see
+    # ``load_subagents_config_from_dict``).
+    _token_budget_is_default: bool = True
+
+    def __init__(self, **data):
+        super().__init__(**data)
+        self._token_budget_is_default = "token_budget" not in self.model_fields_set
+
     def get_timeout_for(self, agent_name: str) -> int:
         """Get the effective timeout for a specific agent.
 
@@ -165,7 +195,12 @@ class SubagentsAppConfig(BaseModel):
             return override.skills
         return None
 
-    def get_token_budget_for(self, agent_name: str) -> TokenBudgetConfig:
+    def get_token_budget_for(
+        self,
+        agent_name: str,
+        *,
+        summarization_enabled: bool = False,
+    ) -> TokenBudgetConfig:
         """Get the effective token-budget config for a specific agent.
 
         Unlike ``max_turns``/``timeout_seconds`` (which keep a custom agent's
@@ -173,10 +208,21 @@ class SubagentsAppConfig(BaseModel):
         every subagent unless explicitly disabled — so the per-agent override
         wins when set, otherwise the global default applies to built-in AND
         custom agents alike (#3875 Phase 2 / umbrella #3857 point 4).
+
+        ``summarization_enabled`` couples the DEFAULT ceiling to whether
+        subagent summarization is on (#3875 Phase 3 review): 1M when
+        compaction is running, 2M otherwise. It ONLY affects the default —
+        any explicitly configured ``token_budget`` (global or per-agent)
+        wins regardless, so a deployment that pinned a value is never
+        silently changed by flipping the summarization switch.
         """
         override = self.agents.get(agent_name)
         if override is not None and override.token_budget is not None:
             return override.token_budget
+        # Only recompute when the caller is using the default (no explicit
+        # global token_budget was set). A user-set global is respected as-is.
+        if self._token_budget_is_default:
+            return default_subagent_token_budget(summarization_enabled=summarization_enabled)
         return self.token_budget
 
 
@@ -191,6 +237,18 @@ def get_subagents_app_config() -> SubagentsAppConfig:
 def load_subagents_config_from_dict(config_dict: dict) -> None:
     """Load subagents configuration from a dictionary."""
     global _subagents_config
+    # The app-config reload path (app_config.py) round-trips via
+    # ``config.subagents.model_dump()``, which serializes a default
+    # ``token_budget`` into the dict. Re-constructing from that dict would make
+    # ``model_fields_set`` contain ``token_budget`` and flip
+    # ``_token_budget_is_default`` to False — breaking the
+    # summarization-coupled recompute in ``get_token_budget_for`` (#3875 Phase
+    # 3). Drop the key when its value still equals the no-compaction default so
+    # the default_factory fires on reconstruction and the "user did not set
+    # this" signal is preserved.
+    tb = config_dict.get("token_budget")
+    if tb is not None and tb == default_subagent_token_budget(summarization_enabled=False).model_dump():
+        config_dict = {k: v for k, v in config_dict.items() if k != "token_budget"}
     _subagents_config = SubagentsAppConfig(**config_dict)
 
     overrides_summary = {}
diff --git a/backend/packages/harness/deerflow/subagents/step_events.py b/backend/packages/harness/deerflow/subagents/step_events.py
index f72e859d7..26f77db04 100644
--- a/backend/packages/harness/deerflow/subagents/step_events.py
+++ b/backend/packages/harness/deerflow/subagents/step_events.py
@@ -95,8 +95,27 @@ def capture_new_step_messages(
     grow, re-examine only the trailing message so an id-less in-place replacement
     (same length, new content) is still captured — ``capture_step_message``'s
     dedup makes an unchanged re-yield a no-op. Returns the new cursor.
+
+    When the history *contracted* (``total < processed_count``) — which happens
+    when ``DeerFlowSummarizationMiddleware`` rewrites the channel via
+    ``RemoveMessage(id=REMOVE_ALL_MESSAGES)`` (#3875 Phase 3) — reset the cursor
+    to the new tail and let ``capture_step_message``'s id/content dedup prevent
+    re-emitting steps captured before the compaction. Without this reset, every
+    step appended after the compaction point is dropped until ``total`` overtakes
+    the stale cursor.
+
+    INVARIANT: after the reset the no-growth branch only re-examines
+    ``messages[-1]``, so a genuinely new AIMessage/ToolMessage inserted at an
+    index BELOW the reset cursor in a compacted list would be missed. This is
+    not reachable today: the summarization middleware puts the summary into a
+    separate ``summary_text`` state key, and the messages channel after
+    compaction holds only already-seen preserved tail messages — compaction
+    never inserts a NEW capturable message below the cursor. If a future
+    middleware violates this invariant, the reset branch needs a full re-scan.
     """
     total = len(messages)
+    if total < processed_count:
+        processed_count = total
     if total > processed_count:
         for message in messages[processed_count:total]:
             capture_step_message(message, captured, seen_ids)
diff --git a/backend/tests/test_subagent_executor.py b/backend/tests/test_subagent_executor.py
index 2cc00c00f..af30bcc3e 100644
--- a/backend/tests/test_subagent_executor.py
+++ b/backend/tests/test_subagent_executor.py
@@ -841,6 +841,60 @@ class TestAsyncExecutionPath:
 
         assert [m["id"] for m in result.ai_messages] == ["ai-1", "tool-1", "tool-2", "tool-3", "ai-2"]
 
+    @pytest.mark.anyio
+    async def test_aexecute_step_capture_survives_history_contraction(self, classes, base_config, mock_agent, msg):
+        """Regression for #3875 Phase 3: DeerFlowSummarizationMiddleware rewrites the
+        messages channel mid-run via ``RemoveMessage(id=REMOVE_ALL_MESSAGES)``,
+        so a later ``values`` snapshot hands the executor a SHORTER message list
+        than the cursor it was tracking. Without the contraction reset in
+        ``capture_new_step_messages``, every step appended after the compaction
+        is dropped until the list length overtakes the stale cursor.
+
+        Faithful to the real middleware: compaction puts the summary into a
+        SEPARATE ``summary_text`` state key — the messages channel after
+        compaction holds only the preserved recent tail (already-seen
+        messages), NOT a synthetic summary AIMessage. So the contraction chunk
+        is the already-seen tail (deduped, no new step); the real regression
+        coverage is that POST-compaction growth is still captured."""
+        SubagentExecutor = classes["SubagentExecutor"]
+
+        human = msg.human("Task")
+        ai1 = msg.ai("turn one", "ai-1")
+        tool1 = msg.tool("r1", "call_1", name="web_search", msg_id="tool-1")
+        ai2 = msg.ai("turn two", "ai-2")  # also the preserved tail after compaction
+        tool2 = msg.tool("r2", "call_2", name="read_file", msg_id="tool-2")
+        final = msg.ai("final answer", "ai-3")
+
+        chunks = [
+            # Pre-compaction growth (cursor → 4).
+            {"messages": [human, ai1]},
+            {"messages": [human, ai1, tool1]},
+            {"messages": [human, ai1, tool1, ai2]},
+            # Compaction: channel rewrites to just the preserved tail (ai2) —
+            # length drops from 4 to 1, below the cursor. ai2 is already seen
+            # (deduped), so no new step is emitted. (The summary lives in
+            # summary_text, out of channel.)
+            {"messages": [ai2]},
+            # Post-compaction growth — the bug: tool-2/final were dropped.
+            {"messages": [ai2, tool2]},
+            {"messages": [ai2, tool2, final]},
+        ]
+        mock_agent.astream = lambda *args, **kwargs: async_iterator(chunks)
+
+        executor = SubagentExecutor(config=base_config, tools=[], thread_id="test-thread")
+        with patch.object(executor, "_create_agent", return_value=mock_agent):
+            result = await executor._aexecute("Task")
+
+        # Pre-compaction steps survive (ai2 not re-emitted — deduped), and
+        # crucially the post-compaction tool + final answer are NOT dropped.
+        assert [m["id"] for m in result.ai_messages] == [
+            "ai-1",
+            "tool-1",
+            "ai-2",
+            "tool-2",
+            "ai-3",
+        ]
+
     @pytest.mark.anyio
     async def test_aexecute_handles_list_content(self, classes, base_config, mock_agent, msg):
         """Test handling of list-type content in AIMessage."""
diff --git a/backend/tests/test_subagent_step_events.py b/backend/tests/test_subagent_step_events.py
index 4193d4598..38423c5e9 100644
--- a/backend/tests/test_subagent_step_events.py
+++ b/backend/tests/test_subagent_step_events.py
@@ -243,6 +243,52 @@ def test_capture_new_step_messages_is_noop_on_values_reyield():
     assert len(captured) == 1
 
 
+def test_capture_new_step_messages_handles_history_contraction():
+    # Regression for #3875 Phase 3: DeerFlowSummarizationMiddleware rewrites the
+    # messages channel via RemoveMessage(id=REMOVE_ALL_MESSAGES), which shrinks
+    # len(messages) below the cursor we were tracking. Without a contraction
+    # reset, every step appended AFTER the compaction is dropped until total
+    # overtakes the stale cursor.
+    #
+    # Faithful to the real middleware: compaction puts the summary into a
+    # SEPARATE ``summary_text`` state key — the messages channel after
+    # compaction holds only the preserved recent tail (already-seen messages),
+    # NOT a synthetic summary AIMessage. So the contraction chunk is the
+    # already-seen tail, deduped by id; the real regression coverage is that
+    # POST-compaction growth is still captured.
+    captured: list[dict] = []
+    seen: set[str] = set()
+
+    # Pre-compaction: a normal growing turn captures 3 steps (cursor → 4).
+    ai1 = AIMessage(content="searching", id="ai-1")
+    tool1 = ToolMessage(content="r1", tool_call_id="c1", name="web_search", id="tool-1")
+    ai2 = AIMessage(content="done turn", id="ai-2")
+    before = [HumanMessage(content="do research", id="h-1"), ai1, tool1, ai2]
+    processed = capture_new_step_messages(before, captured, seen, 0)
+    assert processed == 4
+    assert [c["id"] for c in captured] == ["ai-1", "tool-1", "ai-2"]
+
+    # Compaction rewrites the channel to just the preserved tail (ai2) —
+    # length drops from 4 to 1, below the cursor. ai2 is already seen, so the
+    # dedup makes it a no-op; no new step is emitted. (The summary itself lives
+    # in summary_text and is never a capturable AIMessage — see step_events.py
+    # INVARIANT.)
+    compacted = [ai2]
+    processed = capture_new_step_messages(compacted, captured, seen, processed)
+    assert processed == 1
+    assert [c["id"] for c in captured] == ["ai-1", "tool-1", "ai-2"]  # unchanged
+
+    # Post-compaction growth: a new turn appends after the preserved tail. This
+    # is the bug the fix targets — without the reset, processed_count stays at
+    # 4, total (3) never exceeds it, and tool-2/ai-3 are silently dropped.
+    tool2 = ToolMessage(content="r2", tool_call_id="c2", name="read_file", id="tool-2")
+    ai3 = AIMessage(content="final answer", id="ai-3")
+    after = [ai2, tool2, ai3]
+    processed = capture_new_step_messages(after, captured, seen, processed)
+    assert processed == 3
+    assert [c["id"] for c in captured] == ["ai-1", "tool-1", "ai-2", "tool-2", "ai-3"]
+
+
 def test_run_event_for_task_started():
     record = subagent_run_event({"type": "task_started", "task_id": "call_1", "description": "research X"})
 
diff --git a/backend/tests/test_subagent_timeout_config.py b/backend/tests/test_subagent_timeout_config.py
index d68e0f99f..506f119fd 100644
--- a/backend/tests/test_subagent_timeout_config.py
+++ b/backend/tests/test_subagent_timeout_config.py
@@ -14,6 +14,7 @@ import pytest
 from deerflow.config.subagents_config import (
     SubagentOverrideConfig,
     SubagentsAppConfig,
+    default_subagent_token_budget,
     get_subagents_app_config,
     load_subagents_config_from_dict,
 )
@@ -124,6 +125,57 @@ class TestSubagentsAppConfigDefaults:
         with pytest.raises(ValueError):
             SubagentsAppConfig(max_turns=-60)
 
+    def test_default_token_budget_coupled_to_summarization_switch(self):
+        """The token-budget backstop engages by default (#3857 point 4). Its
+        ``max_tokens`` ceiling is coupled to whether subagent summarization is
+        on (#3875 Phase 3 review): 1M when compaction runs, 2M otherwise —
+        because Phase 2 acknowledged legitimate deep-research runs can exceed
+        1M without compaction, so tightening to 1M unconditionally would
+        prematurely cap summarization-off deployments."""
+        # Default (no summarization): 2M — preserves Phase 2 headroom.
+        budget_off = default_subagent_token_budget(summarization_enabled=False)
+        assert budget_off.enabled is True
+        assert budget_off.max_tokens == 2_000_000
+        assert budget_off.warn_threshold == 0.7
+        # Summarization on: 1M — compaction justifies the tighter ceiling.
+        budget_on = default_subagent_token_budget(summarization_enabled=True)
+        assert budget_on.max_tokens == 1_000_000
+        # The AppConfig model-level default cannot read summarization.enabled,
+        # so it falls back to the no-compaction 2M; the builder recomputes via
+        # get_token_budget_for(summarization_enabled=...).
+        config = SubagentsAppConfig()
+        assert config.token_budget.max_tokens == 2_000_000
+        assert config.token_budget.enabled is True
+
+    def test_get_token_budget_for_couples_default_to_summarization(self):
+        """``get_token_budget_for`` must re-couple the DEFAULT ceiling to the
+        summarization switch, but a user-set budget (global or per-agent) must
+        always win regardless of the switch (#3875 Phase 3 review)."""
+        # Default global budget → re-coupled.
+        config = SubagentsAppConfig()
+        assert config.get_token_budget_for("general-purpose", summarization_enabled=True).max_tokens == 1_000_000
+        assert config.get_token_budget_for("general-purpose", summarization_enabled=False).max_tokens == 2_000_000
+
+    def test_get_token_budget_for_respects_explicit_global(self):
+        """A user-set global ``token_budget`` is respected as-is — the
+        summarization coupling only affects the default."""
+        from deerflow.config.token_budget_config import TokenBudgetConfig
+
+        config = SubagentsAppConfig(token_budget=TokenBudgetConfig(enabled=True, max_tokens=500_000))
+        # Explicit global wins for an agent with no per-agent override.
+        assert config.get_token_budget_for("general-purpose", summarization_enabled=True).max_tokens == 500_000
+        assert config.get_token_budget_for("general-purpose", summarization_enabled=False).max_tokens == 500_000
+
+    def test_get_token_budget_for_respects_per_agent_override(self):
+        """A per-agent ``token_budget`` override wins over both the default and
+        the summarization coupling."""
+        from deerflow.config.token_budget_config import TokenBudgetConfig
+
+        config = SubagentsAppConfig(
+            agents={"bash": SubagentOverrideConfig(token_budget=TokenBudgetConfig(enabled=True, max_tokens=300_000))},
+        )
+        assert config.get_token_budget_for("bash", summarization_enabled=True).max_tokens == 300_000
+
 
 # ---------------------------------------------------------------------------
 # SubagentsAppConfig resolution helpers
diff --git a/backend/tests/test_summarization_middleware.py b/backend/tests/test_summarization_middleware.py
index 6051c8821..7231f432d 100644
--- a/backend/tests/test_summarization_middleware.py
+++ b/backend/tests/test_summarization_middleware.py
@@ -13,9 +13,10 @@ from langgraph.constants import TAG_NOSTREAM
 
 from deerflow.agents.memory.summarization_hook import memory_flush_hook
 from deerflow.agents.middlewares.dynamic_context_middleware import _DYNAMIC_CONTEXT_REMINDER_KEY, DynamicContextMiddleware, is_dynamic_context_reminder
-from deerflow.agents.middlewares.summarization_middleware import DeerFlowSummarizationMiddleware, SummarizationEvent
+from deerflow.agents.middlewares.summarization_middleware import DeerFlowSummarizationMiddleware, SummarizationEvent, create_summarization_middleware
 from deerflow.agents.thread_state import ThreadState
 from deerflow.config.memory_config import MemoryConfig
+from deerflow.config.summarization_config import SummarizationConfig
 
 
 def _messages() -> list:
@@ -629,3 +630,42 @@ def test_multiple_id_swap_triplets_preserve_chronological_order() -> None:
         f"{base2}__memory",
         f"{base2}__user",
     ]
+
+
+def test_factory_attaches_memory_flush_hook_by_default(monkeypatch):
+    """The lead path keeps ``memory_flush_hook`` so pre-compaction messages
+    persist into durable memory. Verified via the factory with memory enabled
+    and the default ``skip_memory_flush=False``."""
+    fake_model = MagicMock()
+    fake_model.with_config.return_value = fake_model
+    monkeypatch.setattr("deerflow.agents.middlewares.summarization_middleware.create_chat_model", lambda **kw: fake_model)
+
+    app_config = SimpleNamespace(
+        summarization=SummarizationConfig(enabled=True),
+        memory=MemoryConfig(enabled=True),
+    )
+    middleware = create_summarization_middleware(app_config=app_config)
+
+    assert middleware is not None
+    assert memory_flush_hook in middleware._before_summarization_hooks
+
+
+def test_factory_skip_memory_flush_omits_hook(monkeypatch):
+    """``skip_memory_flush=True`` (the subagent path) must omit
+    ``memory_flush_hook``: subagents share the parent's ``thread_id``, so
+    without skipping the hook a subagent's internal turns would flush into the
+    PARENT thread's durable memory (#3875 Phase 3 review)."""
+    fake_model = MagicMock()
+    fake_model.with_config.return_value = fake_model
+    monkeypatch.setattr("deerflow.agents.middlewares.summarization_middleware.create_chat_model", lambda **kw: fake_model)
+
+    app_config = SimpleNamespace(
+        summarization=SummarizationConfig(enabled=True),
+        memory=MemoryConfig(enabled=True),
+    )
+    middleware = create_summarization_middleware(app_config=app_config, skip_memory_flush=True)
+
+    assert middleware is not None
+    # memory.enabled is True but the hook is skipped — the whole point.
+    assert memory_flush_hook not in middleware._before_summarization_hooks
+    assert middleware._before_summarization_hooks == []
diff --git a/backend/tests/test_tool_error_handling_middleware.py b/backend/tests/test_tool_error_handling_middleware.py
index d1675cf1f..a7721913c 100644
--- a/backend/tests/test_tool_error_handling_middleware.py
+++ b/backend/tests/test_tool_error_handling_middleware.py
@@ -628,6 +628,57 @@ def test_subagent_runtime_middlewares_place_loop_detection_before_safety_finish(
     assert loop_idx < safety_idx
 
 
+def test_subagent_runtime_middlewares_attach_summarization_when_enabled(monkeypatch):
+    """Subagents must inherit the lead's DeerFlowSummarizationMiddleware so a
+    long-running deep-research subagent compacts its context before it
+    accumulates pathological input (#3875 Phase 3). Gated on the SAME
+    ``summarization.enabled`` switch the lead reads — one config covers both
+    chains. The factory is patched at its source module so the test does not
+    depend on a real chat model; only the gating + wiring is asserted."""
+    from deerflow.agents.middlewares import summarization_middleware as sm
+
+    sentinel = object()
+    captured: dict[str, object] = {}
+
+    def fake_create_summarization_middleware(*, app_config=None, keep=None, skip_memory_flush=False):
+        captured["app_config"] = app_config
+        captured["keep"] = keep
+        captured["skip_memory_flush"] = skip_memory_flush
+        return sentinel
+
+    # summarization is enabled by default False; flip it on so the factory path
+    # is taken (the factory early-returns None when disabled).
+    from deerflow.config.summarization_config import SummarizationConfig
+
+    app_config = _make_app_config().model_copy(update={"summarization": SummarizationConfig(enabled=True)})
+    monkeypatch.setattr(sm, "create_summarization_middleware", fake_create_summarization_middleware)
+    _stub_runtime_middleware_imports(monkeypatch)
+
+    middlewares = build_subagent_runtime_middlewares(app_config=app_config, model_name="test-model")
+
+    # The shared factory received the same app_config the builder did (no lead
+    # wrapper, no config drift between the two chains).
+    assert captured["app_config"] is app_config
+    # skip_memory_flush=True so subagent-internal turns are not flushed into the
+    # PARENT thread's durable memory (#3875 Phase 3 review).
+    assert captured["skip_memory_flush"] is True
+    assert sentinel in middlewares
+
+
+def test_subagent_runtime_middlewares_omit_summarization_when_factory_returns_none(monkeypatch):
+    """When ``summarization.enabled`` is False the shared factory returns None and
+    the subagent chain must NOT carry a summarization middleware — the default
+    state, since SummarizationConfig.enabled defaults to False."""
+    from deerflow.agents.middlewares.summarization_middleware import DeerFlowSummarizationMiddleware
+
+    app_config = _make_app_config()  # summarization.enabled defaults to False
+    _stub_runtime_middleware_imports(monkeypatch)
+
+    middlewares = build_subagent_runtime_middlewares(app_config=app_config, model_name="test-model")
+
+    assert not any(isinstance(m, DeerFlowSummarizationMiddleware) for m in middlewares)
+
+
 def test_lead_runtime_chain_finds_historical_uploads_under_lazy_init_false(tmp_path, monkeypatch):
     """Integration anchor for the ThreadData → Uploads ordering.
 
@@ -680,3 +731,108 @@ def test_lead_runtime_chain_finds_historical_uploads_under_lazy_init_false(tmp_p
     assert "<uploaded_files>" in injected_content
     assert "prior-report.txt" in injected_content
     assert "previous messages" in injected_content  # historical section header
+
+
+def test_subagent_summarization_fires_mid_run_and_produces_usable_result(monkeypatch):
+    """Integration coverage for #3875 Phase 3 review gap: drive the REAL
+    ``DeerFlowSummarizationMiddleware`` (the exact instance the subagent chain
+    gets via ``create_summarization_middleware(skip_memory_flush=True)``) through
+    a ``create_agent`` run, and assert that (a) compaction actually fires mid-run
+    (messages channel contracts via ``RemoveMessage``) and (b) the run still
+    completes with a usable final answer — not just wiring.
+
+    The builder-wiring test above proves the middleware lands on the chain; this
+    proves the live middleware triggers and the run survives it. We bypass the
+    full ``build_subagent_runtime_middlewares`` chain (whose sandbox/thread-data
+    stubs aren't AgentMiddleware-compatible for a live run) and use the factory
+    directly — the same instance the builder appends."""
+    from langchain.agents import create_agent
+    from langchain_core.language_models import BaseChatModel
+    from langchain_core.messages import AIMessage, HumanMessage, RemoveMessage
+    from langchain_core.outputs import ChatGeneration, ChatResult
+
+    from deerflow.agents.middlewares.summarization_middleware import (
+        DeerFlowSummarizationMiddleware,
+        create_summarization_middleware,
+    )
+    from deerflow.agents.thread_state import ThreadState
+    from deerflow.config.memory_config import MemoryConfig
+    from deerflow.config.summarization_config import ContextSize, SummarizationConfig
+
+    # A model that always emits a plain AIMessage — no tools, so the run is a
+    # single turn but the input already exceeds the trigger threshold, forcing
+    # before_model compaction on the first (and only) model call.
+    class _StaticModel(BaseChatModel):
+        text: str = "final answer after compaction"
+
+        @property
+        def _llm_type(self) -> str:
+            return "static"
+
+        def bind_tools(self, tools, **kwargs):
+            return self
+
+        def _generate(self, messages, stop=None, run_manager=None, **kwargs):
+            return ChatResult(generations=[ChatGeneration(message=AIMessage(content=self.text))])
+
+    static_model = _StaticModel()
+    # The factory resolves its summary model via create_chat_model; point it at
+    # the same static model so no real provider is contacted.
+    monkeypatch.setattr(
+        "deerflow.agents.middlewares.summarization_middleware.create_chat_model",
+        lambda **kwargs: static_model,
+    )
+
+    app_config = SimpleNamespace(
+        summarization=SummarizationConfig(
+            enabled=True,
+            trigger=ContextSize(type="messages", value=4),
+            keep=ContextSize(type="messages", value=2),
+        ),
+        # memory disabled + skip_memory_flush=True mirrors the subagent path:
+        # no memory_flush_hook is attached.
+        memory=MemoryConfig(enabled=False),
+    )
+    middleware = create_summarization_middleware(
+        app_config=app_config,
+        skip_memory_flush=True,
+    )
+    assert isinstance(middleware, DeerFlowSummarizationMiddleware), "the real middleware must be built"
+    # Subagent invariant: skip_memory_flush means no durable-memory hook.
+    assert not middleware._before_summarization_hooks
+
+    agent = create_agent(
+        model=static_model,
+        tools=[],
+        middleware=[middleware],
+        state_schema=ThreadState,
+    )
+
+    # 6 messages > trigger(4) → compaction must fire in before_model.
+    seed = [
+        HumanMessage(content="q1", id="h1"),
+        AIMessage(content="a1", id="a1"),
+        HumanMessage(content="q2", id="h2"),
+        AIMessage(content="a2", id="a2"),
+        HumanMessage(content="q3", id="h3"),
+        AIMessage(content="a3", id="a3"),
+    ]
+    chunks = list(agent.stream({"messages": seed}, stream_mode="updates"))
+
+    # (a) Compaction fired: the middleware's before_model emitted a summary + RemoveMessage.
+    before_model_chunks = [c for c in chunks if "DeerFlowSummarizationMiddleware.before_model" in c]
+    assert before_model_chunks, "summarization before_model must fire when messages exceed the trigger"
+    summary_update = before_model_chunks[0]["DeerFlowSummarizationMiddleware.before_model"]
+    assert summary_update.get("summary_text"), "a summary must be produced"
+    emitted = summary_update["messages"]
+    assert isinstance(emitted[0], RemoveMessage), "compaction must lead with RemoveMessage"
+
+    # (b) The run completed with a usable final AIMessage despite compaction.
+    # The model's output surfaces under the "model" node key in updates mode.
+    final_messages: list = []
+    for chunk in chunks:
+        node_msg = chunk.get("model") or chunk.get("agent") or {}
+        final_messages = node_msg.get("messages", final_messages)
+    ai_finals = [m for m in final_messages if isinstance(m, AIMessage)]
+    assert ai_finals, "the run must produce a final AIMessage after compaction"
+    assert ai_finals[-1].content == "final answer after compaction"