diff --git a/backend/packages/harness/deerflow/agents/lead_agent/prompt.py b/backend/packages/harness/deerflow/agents/lead_agent/prompt.py
index ef7917104..2e4d32cec 100644
--- a/backend/packages/harness/deerflow/agents/lead_agent/prompt.py
+++ b/backend/packages/harness/deerflow/agents/lead_agent/prompt.py
@@ -542,6 +542,14 @@ combined with a FastAPI gateway for REST API access [citation:FastAPI](https://f
 {subagent_reminder}- Skill First: Always load the relevant skill before starting **complex** tasks.
 - Progressive Loading: Load resources incrementally as referenced in skills
 - Output Files: Final deliverables must be in `/mnt/user-data/outputs`
+- File Editing Workflow: When revising an existing file, prefer
+  `str_replace` over `write_file` — it sends only the diff and avoids
+  re-emitting the whole file (mirrors Claude Code's Edit and Codex's
+  apply_patch). When writing long new content from scratch, split it
+  into sections: the first `write_file` call creates the file, then use
+  `write_file` with append=True to extend it section by section. This
+  keeps each tool call small and avoids mid-stream chunk-gap timeouts
+  on oversized single-shot writes. (See issue #3189.)  
 - Clarity: Be direct and helpful, avoid unnecessary meta-commentary
 - Including Images and Mermaid: Images and Mermaid diagrams are always welcomed in the Markdown format, and you're encouraged to use `![Image Description](image_path)\n\n` or "```mermaid" to display images in response or Markdown files
 - Multi-task: Better utilize parallel tool calling to call multiple tools at one time for better performance
diff --git a/backend/packages/harness/deerflow/agents/middlewares/llm_error_handling_middleware.py b/backend/packages/harness/deerflow/agents/middlewares/llm_error_handling_middleware.py
index a489d90d0..4ec3b7fe6 100644
--- a/backend/packages/harness/deerflow/agents/middlewares/llm_error_handling_middleware.py
+++ b/backend/packages/harness/deerflow/agents/middlewares/llm_error_handling_middleware.py
@@ -62,6 +62,41 @@ _AUTH_PATTERNS = (
     "未授权",
 )
 
+# Per-exception retry budget overrides.
+#
+# Some transient errors are retriable in principle but expensive to retry at
+# the default budget. StreamChunkTimeoutError in particular fires after the
+# upstream provider has already stalled for `stream_chunk_timeout` seconds
+# (typically 120-240s); a full 3-attempt loop can therefore stack 6-12 minutes
+# of dead air before surfacing the failure to the user. We keep exactly one
+# retry (cheap reconnect that catches genuine transient TCP blips) and then
+# fail fast — the same buffered payload is overwhelmingly likely to fail
+# again at the upstream provider for the same reason.
+#
+# Keys are exception class *names* (not classes) so we don't introduce
+# import-time coupling on optional dependencies like langchain-openai. The
+# value is the absolute max attempt count, NOT additional retries — so a
+# value of 2 means "1 first attempt + 1 retry" (the CR-requested
+# "keep one retry" behavior).
+_RETRY_BUDGET_OVERRIDES: dict[str, int] = {
+    "StreamChunkTimeoutError": 2,
+}
+
+# Exception class names that indicate the upstream stream-chunk watchdog
+# fired because the model stalled mid-flight. These deserve a more specific
+# user-facing message than the generic "temporarily unavailable" copy,
+# because the typical root cause is a long tool-call serialization stalling
+# the upstream stream — and the most actionable advice we can give the user
+# is "ask for a shorter / split output" rather than "wait and retry".
+# Generic connection drops (httpx RemoteProtocolError / ReadError) are
+# intentionally excluded: they routinely fire on transient network blips
+# with normal payloads, where the "split the work" guidance is misleading.
+_STREAM_DROP_EXCEPTIONS: frozenset[str] = frozenset(
+    {
+        "StreamChunkTimeoutError",
+    }
+)
+
 
 class LLMErrorHandlingMiddleware(AgentMiddleware[AgentState]):
     """Retry transient LLM errors and surface graceful assistant messages."""
@@ -83,6 +118,18 @@ class LLMErrorHandlingMiddleware(AgentMiddleware[AgentState]):
         self._circuit_state = "closed"
         self._circuit_probe_in_flight = False
 
+    def _max_attempts_for(self, exc: BaseException) -> int:
+        """Return the effective max attempt count for this exception.
+
+        Falls back to `self.retry_max_attempts` unless the exception class name
+        appears in the per-exception override table.
+        """
+        override = _RETRY_BUDGET_OVERRIDES.get(type(exc).__name__)
+        if override is None:
+            return self.retry_max_attempts
+
+        return min(override, self.retry_max_attempts)
+
     def _check_circuit(self) -> bool:
         """Returns True if circuit is OPEN (fast fail), False otherwise."""
         with self._circuit_lock:
@@ -153,6 +200,7 @@ class LLMErrorHandlingMiddleware(AgentMiddleware[AgentState]):
             "InternalServerError",
             "ReadError",  # httpx.ReadError: connection dropped mid-stream
             "RemoteProtocolError",  # httpx: server closed connection unexpectedly
+            "StreamChunkTimeoutError",  # langchain-openai: chunk gap exceeded stream_chunk_timeout
         }:
             return True, "transient"
         if status_code in _RETRIABLE_STATUS_CODES:
@@ -202,6 +250,20 @@ class LLMErrorHandlingMiddleware(AgentMiddleware[AgentState]):
         if reason == "auth":
             return "The configured LLM provider rejected the request because authentication or access is invalid. Please check the provider credentials and try again."
         if reason in {"busy", "transient"}:
+            # Stream-drop failures (chunk-gap timeout, peer-closed connection,
+            # raw read error) almost always point at a single oversized
+            # tool-call payload — the model spent so long serializing JSON
+            # arguments that the upstream provider buffered and the stream
+            # gap exceeded `stream_chunk_timeout`. Surfacing this distinct
+            # cause lets the user split or shorten their next request
+            # instead of helplessly retrying the same prompt.
+            if type(exc).__name__ in _STREAM_DROP_EXCEPTIONS:
+                return (
+                    "The model's streaming response was interrupted before it could "
+                    "finish. This usually happens when a single response or tool call "
+                    "is very large — please ask the assistant to split the work into "
+                    "smaller steps, or shorten the requested output, and try again."
+                )
             return "The configured LLM provider is temporarily unavailable after multiple retries. Please wait a moment and continue the conversation."
         return f"LLM request failed: {detail}"
 
@@ -259,7 +321,8 @@ class LLMErrorHandlingMiddleware(AgentMiddleware[AgentState]):
                 raise
             except Exception as exc:
                 retriable, reason = self._classify_error(exc)
-                if retriable and attempt < self.retry_max_attempts:
+                max_attempts = self._max_attempts_for(exc)
+                if retriable and attempt < max_attempts:
                     wait_ms = self._build_retry_delay_ms(attempt, exc)
                     logger.warning(
                         "Transient LLM error on attempt %d/%d; retrying in %dms: %s",
@@ -310,7 +373,8 @@ class LLMErrorHandlingMiddleware(AgentMiddleware[AgentState]):
                 raise
             except Exception as exc:
                 retriable, reason = self._classify_error(exc)
-                if retriable and attempt < self.retry_max_attempts:
+                max_attempts = self._max_attempts_for(exc)
+                if retriable and attempt < max_attempts:
                     wait_ms = self._build_retry_delay_ms(attempt, exc)
                     logger.warning(
                         "Transient LLM error on attempt %d/%d; retrying in %dms: %s",
diff --git a/backend/packages/harness/deerflow/config/model_config.py b/backend/packages/harness/deerflow/config/model_config.py
index e9a3e1c16..b747eec99 100644
--- a/backend/packages/harness/deerflow/config/model_config.py
+++ b/backend/packages/harness/deerflow/config/model_config.py
@@ -32,6 +32,16 @@ class ModelConfig(BaseModel):
         description="Extra settings to be passed to the model when thinking is disabled",
     )
     supports_vision: bool = Field(default_factory=lambda: False, description="Whether the model supports vision/image inputs")
+    stream_chunk_timeout: float | None = Field(
+        default=None,
+        description=(
+            "Maximum seconds to wait between successive streaming chunks before "
+            "langchain-openai raises StreamChunkTimeoutError. None means use the "
+            "factory default (240s for OpenAI-compatible clients). Tune higher for "
+            "reasoning models with long thinking pauses; lower for latency-sensitive "
+            "interactive endpoints. Has no effect on non-OpenAI-compatible providers."
+        ),
+    )
     thinking: dict | None = Field(
         default_factory=lambda: None,
         description=(
diff --git a/backend/packages/harness/deerflow/models/factory.py b/backend/packages/harness/deerflow/models/factory.py
index c6a3573f8..57eb15001 100644
--- a/backend/packages/harness/deerflow/models/factory.py
+++ b/backend/packages/harness/deerflow/models/factory.py
@@ -47,6 +47,38 @@ def _enable_stream_usage_by_default(model_use_path: str, model_settings_from_con
         model_settings_from_config["stream_usage"] = True
 
 
+# Default chunk-gap budget for OpenAI-compatible streaming responses.
+#
+# langchain-openai raises ``StreamChunkTimeoutError`` after this many seconds
+# without receiving a chunk. Its own default is 60s, which is too aggressive for
+# reasoning models (DeepSeek-R1, Doubao-thinking, GPT-5) whose first chunk can
+# legitimately take 90~150s. We default to 240s so the streaming layer rarely
+# trips on long thinking pauses; the LLMErrorHandlingMiddleware still retries
+# (budget=2) if a real stall happens. Users can override per-model in config.yaml.
+_DEFAULT_STREAM_CHUNK_TIMEOUT_SECONDS: float = 240.0
+
+
+def _apply_stream_chunk_timeout_default(model_use_path: str, model_settings_from_config: dict) -> None:
+    """Inject a generous ``stream_chunk_timeout`` for OpenAI-compatible clients.
+
+    The ``stream_chunk_timeout`` kwarg is specific to ``langchain_openai:ChatOpenAI``
+    and is rejected by other providers' constructors as an unexpected keyword
+    argument. Behaviour:
+
+    * OpenAI-compatible path: an explicit value in ``config.yaml`` is preserved.
+      An explicit ``null`` is dropped upstream by ``model_dump(exclude_none=True)``
+      and therefore treated as "unset", so the default is injected.
+    * Non-OpenAI path: drop the key so it is never forwarded to an incompatible
+      constructor (which would raise ``TypeError: unexpected keyword argument``).
+    """
+    if model_use_path != "langchain_openai:ChatOpenAI":
+        model_settings_from_config.pop("stream_chunk_timeout", None)
+        return
+    if "stream_chunk_timeout" in model_settings_from_config:
+        return
+    model_settings_from_config["stream_chunk_timeout"] = _DEFAULT_STREAM_CHUNK_TIMEOUT_SECONDS
+
+
 def create_chat_model(name: str | None = None, thinking_enabled: bool = False, *, app_config: AppConfig | None = None, attach_tracing: bool = True, **kwargs) -> BaseChatModel:
     """Create a chat model instance from the config.
 
@@ -128,6 +160,7 @@ def create_chat_model(name: str | None = None, thinking_enabled: bool = False, *
         model_settings_from_config.pop("reasoning_effort", None)
 
     _enable_stream_usage_by_default(model_config.use, model_settings_from_config)
+    _apply_stream_chunk_timeout_default(model_config.use, model_settings_from_config)
 
     # For Codex Responses API models: map thinking mode to reasoning_effort
     from deerflow.models.openai_codex_provider import CodexChatModel
diff --git a/backend/packages/harness/deerflow/sandbox/tools.py b/backend/packages/harness/deerflow/sandbox/tools.py
index 6edc88882..4c04e3ac7 100644
--- a/backend/packages/harness/deerflow/sandbox/tools.py
+++ b/backend/packages/harness/deerflow/sandbox/tools.py
@@ -1,4 +1,5 @@
 import asyncio
+import os
 import posixpath
 import re
 import shlex
@@ -43,6 +44,16 @@ _MAX_GLOB_MAX_RESULTS = 1000
 _DEFAULT_GREP_MAX_RESULTS = 100
 _MAX_GREP_MAX_RESULTS = 500
 _DEFAULT_WRITE_FILE_ERROR_MAX_CHARS = 2000
+
+# Maximum bytes accepted in a single non-append write_file call (issue #3189).
+# Oversized single-shot writes correlate with LLM streaming chunk-gap timeouts
+# because the tool-call JSON payload (which the model must emit as one
+# continuous stream) grows past the safe window. 80 KB ≈ 20K tokens, a
+# comfortable headroom under the factory-default 240s stream_chunk_timeout.
+# Deployments can override via env var DEERFLOW_WRITE_FILE_MAX_BYTES; set to
+# 0 (or negative) to disable the guard entirely.
+_WRITE_FILE_CONTENT_MAX_BYTES = 80 * 1024
+_WRITE_FILE_MAX_BYTES_ENV = "DEERFLOW_WRITE_FILE_MAX_BYTES"
 _LOCAL_BASH_CWD_COMMANDS = {"cd", "pushd"}
 _LOCAL_BASH_COMMAND_WRAPPERS = {"command", "builtin"}
 _LOCAL_BASH_COMMAND_PREFIX_KEYWORDS = {"!", "{", "case", "do", "elif", "else", "for", "if", "select", "then", "time", "until", "while"}
@@ -1671,6 +1682,23 @@ async def _read_file_tool_async(
 read_file_tool.coroutine = _read_file_tool_async
 
 
+def _effective_write_file_max_bytes() -> int:
+    """Return the active size cap for non-append write_file calls.
+
+    Reads ``DEERFLOW_WRITE_FILE_MAX_BYTES`` at call time (not import time)
+    so tests and runtime tweaks take effect without restart. Falls back to
+    the default on missing/malformed values. A non-positive value disables
+    the guard.
+    """
+    raw = os.environ.get(_WRITE_FILE_MAX_BYTES_ENV)
+    if raw is None:
+        return _WRITE_FILE_CONTENT_MAX_BYTES
+    try:
+        return int(raw)
+    except ValueError:
+        return _WRITE_FILE_CONTENT_MAX_BYTES
+
+
 @tool("write_file", parse_docstring=True)
 def write_file_tool(
     runtime: Runtime,
@@ -1679,14 +1707,47 @@ def write_file_tool(
     content: str,
     append: bool = False,
 ) -> str:
-    """Write text content to a file. By default this overwrites the target file; set append to true to add content to the end without replacing existing content.
+    """Write text content to a file. By default this overwrites the target file; set append=True to add content to the end without replacing existing content.
+
+    SIZE POLICY (issue #3189):
+    A single non-append write_file call must not exceed 80 KB of UTF-8 content.
+    Oversized single-shot writes correlate with LLM streaming chunk-gap
+    timeouts because the tool-call JSON payload — which the model must emit as
+    one continuous stream — grows past the safe window. For larger documents,
+    use ONE of these strategies (write_file rejects oversized payloads with an
+    actionable error):
+
+      1. INCREMENTAL EDIT (preferred for revisions): after the initial write,
+         use `str_replace` to surgically update sections. This is the same
+         pattern Claude Code's Write+Edit and OpenAI Codex's apply_patch use,
+         and keeps each tool call's payload small.
+      2. APPEND-IN-CHUNKS (for new long-form content): split the document into
+         sections, each well under 80 KB. First call uses append=False to
+         create the file; subsequent calls use append=True. The 80 KB cap does
+         NOT apply to append=True calls.
+
+    Operators can override the cap via env var `DEERFLOW_WRITE_FILE_MAX_BYTES`
+    (0 disables the guard entirely). Raising it risks streaming timeouts.
 
     Args:
         description: Explain why you are writing to this file in short words. ALWAYS PROVIDE THIS PARAMETER FIRST.
         path: The **absolute** path to the file to write to. ALWAYS PROVIDE THIS PARAMETER SECOND.
         content: The content to write to the file. ALWAYS PROVIDE THIS PARAMETER THIRD.
-        append: Whether to append content to the end of the file instead of overwriting it. Defaults to false.
+        append: Whether to append content to the end of the file instead of overwriting it. Defaults to False.
     """
+    if not append:
+        max_bytes = _effective_write_file_max_bytes()
+        if max_bytes > 0:
+            content_bytes = len(content.encode("utf-8"))
+            if content_bytes > max_bytes:
+                return (
+                    f"Error: write_file content ({content_bytes} bytes) exceeds the "
+                    f"{max_bytes}-byte single-call limit. Split the content into smaller "
+                    "pieces: either (a) write the first section now, then use `str_replace` "
+                    "for further edits, or (b) call write_file again with append=True "
+                    "carrying the next section. See SIZE POLICY in the tool docstring "
+                    "or issue #3189 for the rationale."
+                )
     try:
         requested_path = path
         sandbox = ensure_sandbox_initialized(runtime)
diff --git a/backend/packages/harness/deerflow/subagents/builtins/general_purpose.py b/backend/packages/harness/deerflow/subagents/builtins/general_purpose.py
index 08f0c7593..176194729 100644
--- a/backend/packages/harness/deerflow/subagents/builtins/general_purpose.py
+++ b/backend/packages/harness/deerflow/subagents/builtins/general_purpose.py
@@ -24,6 +24,17 @@ Do NOT use for simple, single-step operations.""",
 - Do NOT ask for clarification - work with the information provided
 </guidelines>
 
+<file_editing_workflow>
+When revising an existing file, prefer `str_replace` over `write_file` —
+it sends only the diff and avoids re-emitting the whole file (mirrors
+Claude Code's Edit and Codex's apply_patch). When writing long new
+content from scratch, split it into sections: the first `write_file`
+call creates the file, then use `write_file` with append=True to extend
+it section by section. This keeps each tool call small and avoids
+mid-stream chunk-gap timeouts on oversized single-shot writes.
+(See issue #3189.)
+</file_editing_workflow>
+
 <output_format>
 When you complete the task, provide:
 1. A brief summary of what was accomplished
diff --git a/backend/tests/test_lead_agent_prompt.py b/backend/tests/test_lead_agent_prompt.py
index a03fa02b5..78a5739f3 100644
--- a/backend/tests/test_lead_agent_prompt.py
+++ b/backend/tests/test_lead_agent_prompt.py
@@ -373,3 +373,44 @@ def test_warm_enabled_skills_cache_logs_on_timeout(monkeypatch, caplog):
 
     assert warmed is False
     assert "Timed out waiting" in caplog.text
+
+
+def test_system_prompt_template_contains_file_editing_workflow_rule():
+    """The File Editing Workflow rule must remain in the system prompt
+    template so the planner picks the right tool (str_replace for edits,
+    write_file + append=True for long new content) and avoids mid-stream
+    chunk-gap timeouts on oversized single-shot writes. See issue #3189
+    / PR #3195.
+
+    We deliberately do NOT assert on any specific byte / word threshold
+    here — that would re-introduce the docstring-lock-in pattern the
+    reviewers flagged. The numeric cap lives in the server-side guard
+    (see test_write_file_tool_size_guard.py), which is where it belongs.
+    """
+    template = prompt_module.SYSTEM_PROMPT_TEMPLATE
+    # Section anchor — keeps the rule discoverable in the assembled prompt.
+    assert "File Editing Workflow" in template
+    # Behavioural anchors — if either of these disappears, the model will
+    # silently regress to single-shot write_file calls for long content.
+    assert "str_replace" in template
+    assert "append=True" in template
+
+
+def test_system_prompt_template_preserves_placeholders():
+    """Ensure the chunking-rule edit didn't drop any f-string placeholder
+    consumed by apply_prompt_template(). A missing placeholder would
+    crash prompt rendering at runtime.
+    """
+    template = prompt_module.SYSTEM_PROMPT_TEMPLATE
+    for ph in (
+        "{agent_name}",
+        "{soul}",
+        "{self_update_section}",
+        "{subagent_thinking}",
+        "{skills_section}",
+        "{deferred_tools_section}",
+        "{subagent_section}",
+        "{acp_section}",
+        "{subagent_reminder}",
+    ):
+        assert ph in template, f"placeholder {ph} accidentally removed"
diff --git a/backend/tests/test_llm_error_handling_middleware.py b/backend/tests/test_llm_error_handling_middleware.py
index 9d2c0fa77..7cce9a4cf 100644
--- a/backend/tests/test_llm_error_handling_middleware.py
+++ b/backend/tests/test_llm_error_handling_middleware.py
@@ -373,7 +373,11 @@ def test_sync_read_error_triggers_retry_loop(monkeypatch: pytest.MonkeyPatch) ->
     result = middleware.wrap_model_call(SimpleNamespace(), handler)
 
     assert isinstance(result, AIMessage)
+    # ReadError is a generic connection drop, not a chunk-gap timeout, so
+    # it must fall back to the legacy transient copy rather than the
+    # specialized "split the work into smaller steps" guidance (#3195 CR).
     assert "temporarily unavailable" in result.content
+    assert "streaming response was interrupted" not in result.content
     assert attempts == 3  # exhausted all retries
     assert len(waits) == 2  # slept between attempts 1→2 and 2→3
 
@@ -397,7 +401,11 @@ async def test_async_read_error_triggers_retry_loop(monkeypatch: pytest.MonkeyPa
     result = await middleware.awrap_model_call(SimpleNamespace(), handler)
 
     assert isinstance(result, AIMessage)
+    # ReadError is a generic connection drop, not a chunk-gap timeout, so
+    # it must fall back to the legacy transient copy rather than the
+    # specialized "split the work into smaller steps" guidance (#3195 CR).
     assert "temporarily unavailable" in result.content
+    assert "streaming response was interrupted" not in result.content
     assert attempts == 3  # exhausted all retries
     assert len(waits) == 2  # slept between attempts 1→2 and 2→3
 
@@ -462,3 +470,211 @@ async def test_async_circuit_breaker_trips_and_recovers(monkeypatch: pytest.Monk
     assert result.content == "Success"
     assert middleware._circuit_failure_count == 0  # RESET
     assert middleware._check_circuit() is False
+
+
+class _StreamChunkTimeoutError(Exception):
+    """Local stand-in for langchain_openai's StreamChunkTimeoutError —
+    matched by class name, no langchain-openai import needed (mirrors
+    how this file already stubs httpx.ReadError / RemoteProtocolError).
+    """
+
+
+_StreamChunkTimeoutError.__name__ = "StreamChunkTimeoutError"
+
+
+def test_classify_error_stream_chunk_timeout_is_retriable() -> None:
+    """StreamChunkTimeoutError must be classified as transient/retriable."""
+    middleware = _build_middleware()
+    exc = _StreamChunkTimeoutError("No streaming chunk received for 120.0s (model=mimo-v2.5, chunks_received=58).")
+    exc.__class__.__name__ = "StreamChunkTimeoutError"
+    retriable, reason = middleware._classify_error(exc)
+    assert retriable is True
+    assert reason == "transient"
+
+
+def test_sync_stream_chunk_timeout_retries_once(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Sync handler raising StreamChunkTimeoutError is retried exactly once —
+    the per-exception override caps it at 2 total attempts (1 first call + 1
+    retry) even when retry_max_attempts=3.
+    Same-payload retry on a chunk-gap timeout buffers the same way upstream;
+    a full 3-attempt loop would stack 6-12 minutes of dead air before
+    surfacing failure. We keep one cheap reconnect for genuine transient TCP
+    blips, then surface the failure so the model can re-plan on its next turn.
+    """
+    middleware = _build_middleware(
+        retry_max_attempts=3,
+        retry_base_delay_ms=10,
+        retry_cap_delay_ms=10,
+    )
+    attempts = 0
+    waits: list[float] = []
+    monkeypatch.setattr("time.sleep", lambda d: waits.append(d))
+
+    def handler(_request) -> AIMessage:
+        nonlocal attempts
+        attempts += 1
+        raise _StreamChunkTimeoutError("No streaming chunk received for 120.0s")
+
+    result = middleware.wrap_model_call(SimpleNamespace(), handler)
+
+    assert isinstance(result, AIMessage)
+    assert "streaming response was interrupted" in result.content
+    # Override caps StreamChunkTimeoutError at 2 attempts (1 first call + 1 retry).
+    assert attempts == 2
+    # Exactly one sleep between the first attempt and the single retry.
+    assert len(waits) == 1
+
+
+@pytest.mark.anyio
+async def test_async_stream_chunk_timeout_retries_once(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Async mirror of the sync test: StreamChunkTimeoutError is capped at
+    2 attempts (1 first call + 1 retry) so we don't stack 6-12 minutes of
+    dead air on a same-payload buffering failure.
+    """
+    middleware = _build_middleware(
+        retry_max_attempts=3,
+        retry_base_delay_ms=10,
+        retry_cap_delay_ms=10,
+    )
+    attempts = 0
+    waits: list[float] = []
+
+    async def fake_sleep(d: float) -> None:
+        waits.append(d)
+
+    monkeypatch.setattr(asyncio, "sleep", fake_sleep)
+
+    async def handler(_request) -> AIMessage:
+        nonlocal attempts
+        attempts += 1
+        raise _StreamChunkTimeoutError("No streaming chunk received for 120.0s")
+
+    result = await middleware.awrap_model_call(SimpleNamespace(), handler)
+
+    assert isinstance(result, AIMessage)
+    assert "streaming response was interrupted" in result.content
+    assert attempts == 2
+    # Exactly one sleep between the first attempt and the single retry.
+    assert len(waits) == 1
+
+
+def test_max_attempts_for_returns_override_for_stream_chunk_timeout() -> None:
+    """StreamChunkTimeoutError must use the tightened budget (2 = "keep one retry"),
+    not the default of 3."""
+    middleware = _build_middleware(retry_max_attempts=3)
+    exc = _StreamChunkTimeoutError("upstream stalled")
+    exc.__class__.__name__ = "StreamChunkTimeoutError"
+
+    assert middleware._max_attempts_for(exc) == 2
+
+
+def test_max_attempts_for_falls_back_to_default_for_unlisted_exception() -> None:
+    """ReadError / RemoteProtocolError keep the full retry budget — only
+    StreamChunkTimeoutError pays for stalling upstream for `stream_chunk_timeout`
+    seconds per attempt, so only it gets the tighter cap.
+    """
+    middleware = _build_middleware(retry_max_attempts=3)
+
+    read_err = _ReadError("conn reset")
+    read_err.__class__.__name__ = "ReadError"
+    proto_err = _RemoteProtocolError("peer closed")
+    proto_err.__class__.__name__ = "RemoteProtocolError"
+
+    assert middleware._max_attempts_for(read_err) == 3
+    assert middleware._max_attempts_for(proto_err) == 3
+    assert middleware._max_attempts_for(FakeError("boom")) == 3
+
+
+def test_max_attempts_for_override_never_exceeds_user_cap() -> None:
+    """If the operator lowered retry_max_attempts below the override default,
+    the user-configured cap wins — overrides only ever *tighten*, never loosen.
+    """
+    middleware = _build_middleware(retry_max_attempts=1)
+    exc = _StreamChunkTimeoutError("upstream stalled")
+    exc.__class__.__name__ = "StreamChunkTimeoutError"
+
+    assert middleware._max_attempts_for(exc) == 1
+
+
+def test_user_message_for_stream_chunk_timeout_mentions_split_or_shorten() -> None:
+    """When the retry budget for StreamChunkTimeoutError is exhausted, the user
+    message must guide the user toward splitting / shortening the request
+    instead of suggesting a generic retry. This is the actionable advice
+    Reviewer B asked for in the follow-up CR (issue #3189).
+    """
+    middleware = _build_middleware()
+    exc = _StreamChunkTimeoutError("No streaming chunk received for 120.0s")
+    exc.__class__.__name__ = "StreamChunkTimeoutError"
+
+    message = middleware._build_user_message(exc, reason="transient")
+
+    assert "streaming response was interrupted" in message
+    assert "split" in message or "shorten" in message
+    # The old generic "streaming response was interrupted" wording must NOT appear here,
+    # otherwise the actionable guidance is buried.
+    assert "temporarily unavailable" not in message
+
+
+def test_user_message_for_remote_protocol_error_uses_generic_transient_copy() -> None:
+    """RemoteProtocolError is a generic connection drop that can fire on
+    transient network blips with perfectly normal payloads. The
+    "split the work into smaller steps" guidance only applies when the
+    upstream chunk-gap watchdog fires (StreamChunkTimeoutError), so
+    RemoteProtocolError must fall back to the legacy transient copy.
+    Regression guard for the #3195 CR feedback.
+    """
+    middleware = _build_middleware()
+    exc = _RemoteProtocolError("Server closed connection unexpectedly")
+    exc.__class__.__name__ = "RemoteProtocolError"
+
+    message = middleware._build_user_message(exc, reason="transient")
+
+    assert "temporarily unavailable" in message
+    assert "streaming response was interrupted" not in message
+
+
+def test_user_message_for_read_error_uses_generic_transient_copy() -> None:
+    """httpx.ReadError is symmetric to RemoteProtocolError: a generic
+    connection drop that must NOT receive the "split the work" guidance.
+    Regression guard for the #3195 CR feedback.
+    """
+    middleware = _build_middleware()
+    exc = FakeError("connection dropped mid-stream")
+    exc.__class__.__name__ = "ReadError"
+
+    message = middleware._build_user_message(exc, reason="transient")
+
+    assert "temporarily unavailable" in message
+    assert "streaming response was interrupted" not in message
+
+
+def test_user_message_for_generic_transient_keeps_legacy_copy() -> None:
+    """Generic transient errors (HTTP 503, 'cluster busy', etc.) must keep
+    the original 'streaming response was interrupted' message — only stream-drop
+    exceptions get the new specialized copy. This prevents regression on
+    callers who already rely on the legacy wording.
+    """
+    middleware = _build_middleware()
+    exc = FakeError("server busy", status_code=503)
+
+    message = middleware._build_user_message(exc, reason="transient")
+
+    assert "temporarily unavailable" in message
+    assert "streaming response was interrupted" not in message
+
+
+def test_user_message_for_quota_unchanged() -> None:
+    """Sanity check: the quota / auth branches must remain untouched by the
+    stream-drop refactor.
+    """
+    middleware = _build_middleware()
+    exc = FakeError("insufficient_quota", status_code=429, code="insufficient_quota")
+
+    message = middleware._build_user_message(exc, reason="quota")
+
+    assert "out of quota" in message
+    assert "streaming response was interrupted" not in message
diff --git a/backend/tests/test_model_factory.py b/backend/tests/test_model_factory.py
index f8b84331c..562c8874c 100644
--- a/backend/tests/test_model_factory.py
+++ b/backend/tests/test_model_factory.py
@@ -1069,3 +1069,116 @@ def test_no_duplicate_kwarg_when_reasoning_effort_in_config_and_thinking_disable
 
     # kwargs (runtime) takes precedence: thinking-disabled path sets reasoning_effort=minimal
     assert captured.get("reasoning_effort") == "minimal"
+
+
+# ---------------------------------------------------------------------------
+# stream_chunk_timeout default injection (issue #3189)
+# ---------------------------------------------------------------------------
+
+
+def test_stream_chunk_timeout_defaults_to_240_for_openai_compatible_model(monkeypatch):
+    """OpenAI-compatible clients must receive a generous 240s chunk-gap budget by
+    default, so reasoning models with long thinking pauses don't trip
+    langchain-openai's aggressive 60s built-in default.
+    """
+    model = _make_model(use="langchain_openai:ChatOpenAI")
+    cfg = _make_app_config([model])
+
+    captured: dict = {}
+
+    class CapturingModel(FakeChatModel):
+        def __init__(self, **kwargs):
+            captured.update(kwargs)
+            BaseChatModel.__init__(self, **kwargs)
+
+    _patch_factory(monkeypatch, cfg, model_class=CapturingModel)
+    factory_module.create_chat_model(name="test-model")
+
+    assert captured.get("stream_chunk_timeout") == 240.0
+
+
+def test_stream_chunk_timeout_user_value_not_overridden(monkeypatch):
+    """If the user explicitly sets stream_chunk_timeout in config.yaml, the
+    factory must not overwrite it with the default — even if the value is
+    smaller (60s) or larger (600s) than the default.
+    """
+    model = ModelConfig(
+        name="custom-timeout-model",
+        display_name="Custom Timeout",
+        description=None,
+        use="langchain_openai:ChatOpenAI",
+        model="gpt-4o-mini",
+        stream_chunk_timeout=60.0,  # user-set explicit value
+    )
+    cfg = _make_app_config([model])
+
+    captured: dict = {}
+
+    class CapturingModel(FakeChatModel):
+        def __init__(self, **kwargs):
+            captured.update(kwargs)
+            BaseChatModel.__init__(self, **kwargs)
+
+    _patch_factory(monkeypatch, cfg, model_class=CapturingModel)
+    factory_module.create_chat_model(name="custom-timeout-model")
+
+    assert captured.get("stream_chunk_timeout") == 60.0
+
+
+def test_stream_chunk_timeout_not_injected_for_non_openai_provider(monkeypatch):
+    """Only langchain_openai:ChatOpenAI receives the default. Anthropic / Vertex /
+    other clients that don't understand this kwarg must not be polluted with it.
+    """
+    model = _make_model(use="langchain_anthropic:ChatAnthropic")
+    cfg = _make_app_config([model])
+
+    captured: dict = {}
+
+    class CapturingModel(FakeChatModel):
+        def __init__(self, **kwargs):
+            captured.update(kwargs)
+            BaseChatModel.__init__(self, **kwargs)
+
+    _patch_factory(monkeypatch, cfg, model_class=CapturingModel)
+    factory_module.create_chat_model(name="test-model")
+
+    assert "stream_chunk_timeout" not in captured
+
+
+def test_stream_chunk_timeout_default_constant_is_documented():
+    """Lock the default value at 240s. If we ever want to change this, the
+    deliberate update here (and the docstring on _apply_stream_chunk_timeout_default)
+    forces a paired review of the rationale comment block above the constant.
+    """
+    assert factory_module._DEFAULT_STREAM_CHUNK_TIMEOUT_SECONDS == 240.0
+
+
+def test_stream_chunk_timeout_popped_for_non_openai_provider_when_user_set_it(monkeypatch):
+    """Regression for CR feedback on issue #3189: if a user accidentally sets
+    ``stream_chunk_timeout`` on a non-OpenAI provider, the factory must drop
+    the kwarg before forwarding it to the model constructor. Otherwise the
+    third-party client raises ``TypeError: unexpected keyword argument
+    'stream_chunk_timeout'`` because the parameter is specific to
+    ``langchain_openai:ChatOpenAI``.
+    """
+    model = ModelConfig(
+        name="anthropic-with-stray-timeout",
+        display_name="Anthropic With Stray Timeout",
+        description=None,
+        use="langchain_anthropic:ChatAnthropic",
+        model="claude-sonnet-4",
+        stream_chunk_timeout=60.0,  # user-set on a non-OpenAI provider — must be dropped
+    )
+    cfg = _make_app_config([model])
+
+    captured: dict = {}
+
+    class CapturingModel(FakeChatModel):
+        def __init__(self, **kwargs):
+            captured.update(kwargs)
+            BaseChatModel.__init__(self, **kwargs)
+
+    _patch_factory(monkeypatch, cfg, model_class=CapturingModel)
+    factory_module.create_chat_model(name="anthropic-with-stray-timeout")
+
+    assert "stream_chunk_timeout" not in captured
diff --git a/backend/tests/test_write_file_tool_size_guard.py b/backend/tests/test_write_file_tool_size_guard.py
new file mode 100644
index 000000000..21f20a6bd
--- /dev/null
+++ b/backend/tests/test_write_file_tool_size_guard.py
@@ -0,0 +1,116 @@
+"""Size-guard tests for write_file_tool (issue #3189, PR #3195).
+
+These tests verify that write_file_tool rejects oversized single-shot payloads
+with an actionable message, while leaving append-mode and env-override paths
+untouched. They run purely against the tool's internal guard — no real sandbox
+or filesystem is exercised, so they're fast and hermetic.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from deerflow.sandbox import tools as tools_module
+from deerflow.sandbox.tools import write_file_tool
+
+
+def _call_write_file(*, content: str, append: bool = False) -> str:
+    """Invoke write_file_tool via its underlying callable.
+
+    We patch the sandbox initialisation chain to a no-op MagicMock so the test
+    focuses purely on the size guard. The guard runs BEFORE any sandbox call,
+    so when the guard rejects we never enter the patched path; when the guard
+    passes, the patched sandbox.write_file returns silently and the tool
+    returns "OK".
+    """
+    fn = getattr(write_file_tool, "func", write_file_tool)
+    runtime = MagicMock()
+
+    with (
+        patch.object(tools_module, "ensure_sandbox_initialized") as mock_ensure,
+        patch.object(tools_module, "ensure_thread_directories_exist"),
+        patch.object(tools_module, "is_local_sandbox", return_value=False),
+        patch.object(tools_module, "get_file_operation_lock") as mock_lock,
+    ):
+        sandbox = MagicMock()
+        sandbox.write_file = MagicMock()
+        mock_ensure.return_value = sandbox
+        mock_lock.return_value.__enter__ = MagicMock(return_value=None)
+        mock_lock.return_value.__exit__ = MagicMock(return_value=False)
+
+        return fn(
+            runtime=runtime,
+            description="test write",
+            path="/tmp/test.txt",
+            content=content,
+            append=append,
+        )
+
+
+def test_below_cap_succeeds():
+    """A 79 KB payload sits comfortably under the 80 KB default and must pass
+    straight through to the sandbox layer.
+    """
+    payload = "a" * (79 * 1024)
+    result = _call_write_file(content=payload)
+    assert result == "OK"
+
+
+def test_above_cap_returns_actionable_error():
+    """An 81 KB payload trips the guard. The error message must name the
+    cap, the actual size, and steer the LLM toward str_replace / append=True
+    — these are the exact handles Reviewer A/B asked for in PR #3195.
+    """
+    payload = "a" * (81 * 1024)
+    result = _call_write_file(content=payload)
+
+    assert result.startswith("Error: write_file content")
+    assert "81920 bytes" in result or "82944 bytes" in result, "Error must report the actual content size so the LLM/operator can judge how much to trim or chunk."
+    assert "str_replace" in result, "Error must point to str_replace as the preferred incremental-edit path."
+    assert "append=True" in result, "Error must also surface the append-in-chunks alternative."
+
+
+def test_above_cap_with_append_true_bypasses_guard():
+    """append=True is the *correct* way to write a large document in chunks,
+    so the guard must not block it. The 80 KB cap intentionally applies only
+    to single-shot overwrite calls.
+    """
+    payload = "a" * (200 * 1024)  # 200 KB
+    result = _call_write_file(content=payload, append=True)
+    assert result == "OK", f"append=True must bypass the size guard, got: {result!r}"
+
+
+def test_env_override_raises_cap(monkeypatch: pytest.MonkeyPatch):
+    """Setting DEERFLOW_WRITE_FILE_MAX_BYTES lets deployments accept larger
+    payloads when the underlying LLM/network can demonstrably handle them.
+    """
+    monkeypatch.setenv("DEERFLOW_WRITE_FILE_MAX_BYTES", str(300 * 1024))
+    payload = "a" * (150 * 1024)  # 150 KB — would normally trip the 80 KB cap
+    result = _call_write_file(content=payload)
+    assert result == "OK"
+
+
+def test_env_override_zero_disables_guard(monkeypatch: pytest.MonkeyPatch):
+    """Setting the env var to 0 is the documented escape hatch for operators
+    who want to opt out of the guard entirely (e.g. when running models with
+    very large stream_chunk_timeout values).
+    """
+    monkeypatch.setenv("DEERFLOW_WRITE_FILE_MAX_BYTES", "0")
+    payload = "a" * (500 * 1024)  # 500 KB
+    result = _call_write_file(content=payload)
+    assert result == "OK"
+
+
+def test_env_override_malformed_falls_back_to_default(monkeypatch: pytest.MonkeyPatch):
+    """A typo in the env var (e.g. 'lots') must not crash the tool — fall
+    back silently to the safe 80 KB default. Crashing on every write because
+    of a misconfigured env var would be far worse than ignoring it.
+    """
+    monkeypatch.setenv("DEERFLOW_WRITE_FILE_MAX_BYTES", "lots")
+    # 100 KB should still be rejected because the malformed value falls back
+    # to the 80 KB default.
+    payload = "a" * (100 * 1024)
+    result = _call_write_file(content=payload)
+    assert result.startswith("Error: write_file content")