feat(reasoning): add recover strategy for missing reasoning content (#13)

README.md

@@ -4,7 +4,7 @@ Compatibility proxy connecting Cursor to DeepSeek thinking models (`deepseek-v4-
 
 ## What It Does
 
-- ✅ Caches DeepSeek `reasoning_content` from regular and streamed responses, then restores it on later tool-call turns when Cursor omits it. If the exact original reasoning is unavailable, the proxy fails closed instead of sending a fake placeholder. See [DeepSeek docs](https://api-docs.deepseek.com/guides/thinking_mode#tool-calls) for more details.
+- ✅ Caches DeepSeek `reasoning_content` from regular and streamed responses, then restores it on later tool-call turns when Cursor omits it. If old or mixed-model chat history cannot be repaired exactly, the proxy can recover by continuing from recent context and showing a small Cursor-visible notice. See [DeepSeek docs](https://api-docs.deepseek.com/guides/thinking_mode#tool-calls) for more details.
 - ✅ Mirrors streamed `reasoning_content` into Cursor-visible `<think>...</think>` text so that thinking tokens are shown in Cursor's UI. For BYOK/proxy mode, Cursor renders this as normal text, not as a native collapsible thinking block.
 - ✅ Starts an ngrok tunnel so Cursor can reach the local proxy through a public HTTPS URL.
 - ✅ Provides other compatibility fixes to make DeepSeek models run well in Cursor.
@@ -129,7 +129,7 @@ Select `deepseek-v4-pro` in Cursor and use chat or agent mode as usual.
 
 DeepSeek's [thinking mode](https://api-docs.deepseek.com/guides/thinking_mode#tool-calls) requires `reasoning_content` from assistant messages in tool-call sequences to be passed back in later requests. Cursor may omit this field, causing DeepSeek to return a 400 error. This proxy sits between Cursor and DeepSeek (`Cursor → ngrok → proxy → DeepSeek API`) and repairs requests when it has the exact original reasoning cached.
 
-- Core fix: every DeepSeek response, streaming or non-streaming, has its `reasoning_content` stored in a local SQLite cache keyed by message signature, tool-call ID, and tool-call function signature. On outgoing thinking-mode requests, the proxy restores missing `reasoning_content` for tool-call-related assistant messages and sends the complete history to DeepSeek. If the cache is cold, such as after a proxy restart, it returns a local error instead of fabricating reasoning.
+- Core fix: every DeepSeek response, streaming or non-streaming, has its `reasoning_content` stored in a local SQLite cache keyed by message signature, tool-call ID, and tool-call function signature. On outgoing thinking-mode requests, the proxy restores missing `reasoning_content` for tool-call-related assistant messages and sends the complete history to DeepSeek. If the cache is cold, such as after a proxy restart or model switch, the default recovery mode omits older unrecoverable tool-call history, continues from the latest user request, logs the recovery, and prefixes the next Cursor response with a small notice.
 - Multi-conversation isolation: cache keys are scoped by a SHA-256 hash of the canonical conversation prefix (roles, content, tool calls, excluding `reasoning_content`) plus the upstream model/configuration and an API-key hash. Concurrent or interleaved threads with different histories get different scopes, so reused tool-call IDs do not collide. Byte-identical cloned histories are indistinguishable unless Cursor sends a differentiating history.
 - DeepSeek [prefix caching](https://api-docs.deepseek.com/guides/kv_cache) compatibility: the proxy does not inject synthetic thread IDs, timestamps, or cache-control messages into the prompt. When it restores cached reasoning, it restores the exact original string, preserving repeated prefixes for DeepSeek's automatic best-effort context cache.
 - Additional compatibility fixes: the proxy converts legacy `functions`/`function_call` fields to `tools`/`tool_choice`, preserves required and named tool-choice semantics, normalizes `reasoning_effort` aliases per DeepSeek docs, strips mirrored `<think>` blocks from assistant content, converts multi-part content arrays to plain text, logs DeepSeek prompt-cache usage when available, and mirrors `reasoning_content` into Cursor-visible `<think>...</think>` blocks for thinking display.
@@ -150,15 +150,25 @@ Run without ngrok for local curl testing:
 PROXY_NGROK=false deepseek-cursor-proxy --port 9000 --verbose
 ```
 
-If Cursor shows `missing_reasoning_content`, the current chat contains thinking-mode tool-call history whose original DeepSeek `reasoning_content` is not in the local cache. This commonly happens when continuing an older chat after a proxy restart, cache clear, or cache format/config change. The local 409 response includes a diagnostic placeholder so the cause is visible, but that placeholder is not forwarded to DeepSeek in the default safe mode. Start a new chat, or retry from the original tool-call turn while the proxy is running so it can capture the reasoning.
+If the current chat contains thinking-mode tool-call history whose original DeepSeek `reasoning_content` is not in the local cache, the default `recover` mode avoids hard failure by dropping older unrecoverable tool-call history, forwarding the latest user request with a system recovery note, logging what happened, and prefixing the next assistant response with:
 
-For debugging an old Cursor history, you can opt into a non-compliant compatibility fallback:
-
-```bash
-deepseek-cursor-proxy --verbose --missing-reasoning-strategy placeholder
+```text
+[deepseek-cursor-proxy] Recovered this DeepSeek chat because older tool-call reasoning was unavailable; continuing with recent context only.
 ```
 
-This inserts a loud placeholder into missing `reasoning_content` fields and forwards the request. It may still be rejected by DeepSeek and should not be used for normal work.
+This commonly happens when continuing an older chat after a proxy restart, cache clear, cache format/config change, or switching from another model into DeepSeek. If you run strict debugging mode, the proxy returns `missing_reasoning_content` instead of recovering and the error message tells you to switch back to recover mode.
+
+The recovery strategy is not a config-file setting. For strict DeepSeek API behavior while debugging, pass the runtime flag:
+
+```bash
+deepseek-cursor-proxy --verbose --missing-reasoning-strategy reject
+```
+
+To turn automatic recovery back on, restart without that flag or pass:
+
+```bash
+deepseek-cursor-proxy --verbose --missing-reasoning-strategy recover
+```
 
 Use another config file:
 

src/deepseek_cursor_proxy/config.py

@@ -15,7 +15,10 @@ REASONING_CONTENT_FILE_NAME = "reasoning_content.sqlite3"
 TRUE_VALUES = {"1", "true", "yes", "on"}
 FALSE_VALUES = {"0", "false", "no", "off"}
 MISSING = object()
-DEFAULT_CONFIG_TEXT = """# This file was created automatically at ~/.deepseek-cursor-proxy/config.yaml.
+DEFAULT_CONFIG_HEADER = (
+    "# This file was created automatically at ~/.deepseek-cursor-proxy/config.yaml."
+)
+DEFAULT_CONFIG_TEXT = f"""{DEFAULT_CONFIG_HEADER}
 # API keys are read from Cursor's Authorization header and forwarded upstream.
 
 # `model` is the fallback when a request has no model; Cursor's requested
@@ -35,7 +38,6 @@ max_request_body_bytes: 20971520
 cors: false
 
 reasoning_content_path: reasoning_content.sqlite3
-missing_reasoning_strategy: reject
 reasoning_cache_max_age_seconds: 604800
 reasoning_cache_max_rows: 10000
 """
@@ -76,6 +78,42 @@ def load_config_file(config_path: str | Path) -> dict[str, Any]:
     return dict(loaded)
 
 
+def migrate_default_config_file(
+    settings: dict[str, Any],
+    config_path: Path,
+    live_env: Mapping[str, str],
+    original_config_path: str | Path | None,
+) -> dict[str, Any]:
+    if original_config_path is not None:
+        return settings
+    if "DEEPSEEK_CURSOR_PROXY_CONFIG_PATH" in live_env:
+        return settings
+    if config_path != default_config_path():
+        return settings
+    if "missing_reasoning_strategy" not in settings:
+        return settings
+
+    migrated = dict(settings)
+    migrated.pop("missing_reasoning_strategy", None)
+    try:
+        text = config_path.read_text(encoding="utf-8")
+    except OSError:
+        return migrated
+    if not text.startswith(DEFAULT_CONFIG_HEADER):
+        return settings
+
+    updated_lines = [
+        line
+        for line in text.splitlines()
+        if not line.strip().startswith("missing_reasoning_strategy:")
+    ]
+    updated_text = "\n".join(updated_lines) + "\n"
+    if updated_text != text:
+        config_path.write_text(updated_text, encoding="utf-8")
+        config_path.chmod(0o600)
+    return migrated
+
+
 def resolve_config_path(
     env: Mapping[str, str] | None, config_path: str | Path | None
 ) -> Path:
@@ -150,6 +188,7 @@ def settings_and_env(
     env: Mapping[str, str] | None, config_path: str | Path | None
 ) -> tuple[dict[str, Any], dict[str, str], Path]:
     live_env = dict(os.environ if env is None else env)
+    original_config_path = config_path
     config_path = resolve_config_path(live_env, config_path)
     if (
         config_path == default_config_path()
@@ -157,7 +196,14 @@ def settings_and_env(
         and not config_path.exists()
     ):
         populate_default_config_file(config_path)
-    return load_config_file(config_path), live_env, config_path
+    settings = load_config_file(config_path)
+    settings = migrate_default_config_file(
+        settings,
+        config_path,
+        live_env,
+        original_config_path,
+    )
+    return settings, live_env, config_path
 
 
 @dataclass(frozen=True)
@@ -172,7 +218,7 @@ class ProxyConfig:
     request_timeout: float = 300.0
     max_request_body_bytes: int = 20 * 1024 * 1024
     reasoning_content_path: Path = field(default_factory=default_reasoning_content_path)
-    missing_reasoning_strategy: str = "reject"
+    missing_reasoning_strategy: str = "recover"
     reasoning_cache_max_age_seconds: int = 7 * 24 * 60 * 60
     reasoning_cache_max_rows: int = 10000
     cursor_display_reasoning: bool = True
@@ -207,22 +253,6 @@ class ProxyConfig:
         if thinking not in {"enabled", "disabled", "pass-through"}:
             thinking = "enabled"
 
-        missing_reasoning_strategy = (
-            as_str(
-                setting_value(
-                    settings,
-                    live_env,
-                    "missing_reasoning_strategy",
-                    "MISSING_REASONING_STRATEGY",
-                ),
-                "reject",
-            )
-            .strip()
-            .lower()
-        )
-        if missing_reasoning_strategy not in {"reject", "placeholder"}:
-            missing_reasoning_strategy = "reject"
-
         return cls(
             host=as_str(
                 setting_value(
@@ -307,7 +337,6 @@ class ProxyConfig:
                 default_reasoning_content_path(),
                 config_dir,
             ),
-            missing_reasoning_strategy=missing_reasoning_strategy,
             reasoning_cache_max_age_seconds=as_int(
                 setting_value(
                     settings,

src/deepseek_cursor_proxy/server.py

@@ -3,6 +3,7 @@ from __future__ import annotations
 import argparse
 from dataclasses import replace
 import gzip
+from http.client import HTTPException
 import json
 import logging
 from http.server import BaseHTTPRequestHandler, ThreadingHTTPServer
@@ -24,7 +25,7 @@ from .reasoning_store import ReasoningStore, conversation_scope
 from .streaming import CursorReasoningDisplayAdapter, StreamAccumulator
 from .tunnel import NgrokTunnel, local_tunnel_target
 from .transform import (
-    PLACEHOLDER_REASONING_CONTENT,
+    RECOVERY_NOTICE_CONTENT,
     prepare_upstream_request,
     rewrite_response_body,
 )
@@ -137,22 +138,32 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
                 "restored reasoning_content on %s assistant message(s)",
                 prepared.patched_reasoning_messages,
             )
-        if prepared.placeholder_reasoning_messages:
+        if prepared.recovered_reasoning_messages:
+            if prepared.recovery_notice:
                 LOG.warning(
                     (
-                    "inserted placeholder reasoning_content on %s assistant "
-                    "message(s); this is compatibility mode and may still be "
-                    "rejected by DeepSeek"
+                        "recovered request because cached reasoning_content was "
+                        "unavailable for %s assistant message(s); omitted %s "
+                        "older message(s) from forwarded history and will show "
+                        "a Cursor notice"
                     ),
-                prepared.placeholder_reasoning_messages,
+                    prepared.recovered_reasoning_messages,
+                    prepared.recovery_dropped_messages,
+                )
+            else:
+                LOG.info(
+                    (
+                        "continued recovered request; omitted %s old message(s) "
+                        "before the prior recovery boundary"
+                    ),
+                    prepared.recovery_dropped_messages,
                 )
         if prepared.missing_reasoning_messages:
-            diagnostic_placeholder = (
-                f"{PLACEHOLDER_REASONING_CONTENT} "
-                "[not sent upstream because missing_reasoning_strategy=reject]"
-            )
             LOG.warning(
-                "rejected request path=%s status=409 reason=missing_reasoning_content count=%s",
+                (
+                    "strict missing-reasoning mode rejected request path=%s "
+                    "status=409 reason=missing_reasoning_content count=%s"
+                ),
                 request_path,
                 prepared.missing_reasoning_messages,
             )
@@ -161,26 +172,28 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
                 {
                     "error": {
                         "message": (
-                            "Missing cached DeepSeek reasoning_content for a "
-                            f"thinking-mode tool-call history on "
+                            "deepseek-cursor-proxy is running in strict "
+                            "missing-reasoning mode and cannot automatically "
+                            "recover this thinking-mode tool-call history because "
+                            "cached DeepSeek reasoning_content is missing for "
                             f"{prepared.missing_reasoning_messages} assistant "
-                            "message(s). This usually means the chat has tool-call "
-                            "turns that were not captured by this proxy/cache. Start "
-                            "a new chat or retry from the original tool-call turn."
+                            "message(s). Restart without "
+                            "`--missing-reasoning-strategy reject`, or pass "
+                            "`--missing-reasoning-strategy recover`, so the proxy "
+                            "can recover from partial chat history automatically."
                         ),
                         "type": "missing_reasoning_content",
                         "code": "missing_reasoning_content",
                         "missing_reasoning_messages": prepared.missing_reasoning_messages,
-                        "diagnostic_placeholder": diagnostic_placeholder,
                     }
                 },
             )
             return
         LOG.info(
-            "deepseek send: %s patched=%s placeholder=%s",
+            "deepseek send: %s patched=%s recovered=%s",
             compact_request_stats(prepared.payload),
             prepared.patched_reasoning_messages,
-            prepared.placeholder_reasoning_messages,
+            prepared.recovered_reasoning_messages,
         )
 
         if self.config.verbose:
@@ -252,6 +265,7 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
                     prepared.original_model,
                     prepared.payload["messages"],
                     prepared.cache_namespace,
+                    prepared.recovery_notice,
                 )
             else:
                 sent_response = self._proxy_regular_response(
@@ -259,19 +273,21 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
                     prepared.original_model,
                     prepared.payload["messages"],
                     prepared.cache_namespace,
+                    prepared.recovery_notice,
                 )
             if not sent_response:
                 return
             LOG.info(
                 (
                     "request complete status=%s stream=%s elapsed_ms=%s "
-                    "patched_reasoning=%s missing_reasoning=%s"
+                    "patched_reasoning=%s missing_reasoning=%s recovered_reasoning=%s"
                 ),
                 upstream_status,
                 bool(prepared.payload.get("stream")),
                 elapsed_ms(started),
                 prepared.patched_reasoning_messages,
                 prepared.missing_reasoning_messages,
+                prepared.recovered_reasoning_messages,
             )
 
     def _cursor_authorization(self) -> str | None:
@@ -419,6 +435,7 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
         original_model: str,
         request_messages: list[dict[str, Any]],
         cache_namespace: str,
+        recovery_notice: str | None = None,
     ) -> bool:
         body = read_response_body(response)
         try:
@@ -428,6 +445,7 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
                 self.reasoning_store,
                 request_messages,
                 cache_namespace,
+                content_prefix=recovery_notice,
             )
         except (json.JSONDecodeError, UnicodeDecodeError) as exc:
             LOG.warning("failed to rewrite upstream JSON response: %s", exc)
@@ -457,6 +475,7 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
         original_model: str,
         request_messages: list[dict[str, Any]],
         cache_namespace: str,
+        recovery_notice: str | None = None,
     ) -> bool:
         sent_headers = self._send_response_headers(
             getattr(response, "status", 200),
@@ -479,12 +498,22 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
         )
         scope = conversation_scope(request_messages, cache_namespace)
         finalized = False
+        pending_recovery_notice = recovery_notice
         while True:
+            try:
                 line = response.readline()
+            except (HTTPException, OSError) as exc:
+                LOG.warning("upstream streaming response read failed: %s", exc)
+                return False
             if not line:
                 break
-            rewritten, finalized = self._rewrite_sse_line(
-                line, original_model, accumulator, scope, display_adapter
+            rewritten, finalized, pending_recovery_notice = self._rewrite_sse_line(
+                line,
+                original_model,
+                accumulator,
+                scope,
+                display_adapter,
+                pending_recovery_notice,
             )
             if not self._write_to_client(
                 rewritten, "sending streaming response chunk", flush=True
@@ -508,10 +537,11 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
         accumulator: StreamAccumulator,
         scope: str,
         display_adapter: CursorReasoningDisplayAdapter | None,
-    ) -> tuple[bytes, bool]:
+        recovery_notice: str | None = None,
+    ) -> tuple[bytes, bool, str | None]:
         stripped = line.strip()
         if not stripped.startswith(b"data:"):
-            return line, False
+            return line, False, recovery_notice
 
         data = stripped[len(b"data:") :].strip()
         if data == b"[DONE]":
@@ -520,19 +550,30 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
             stored = accumulator.store_reasoning(self.reasoning_store, scope)
             if stored:
                 LOG.info("stored %s streaming reasoning cache key(s)", stored)
+            prefix = b""
             if display_adapter is None:
-                return b"data: [DONE]\n\n", True
+                if recovery_notice:
+                    prefix += sse_data(
+                        recovery_notice_chunk(original_model, recovery_notice)
+                    )
+                return prefix + b"data: [DONE]\n\n", True, None
             closing_chunk = display_adapter.flush_chunk(original_model)
-            if closing_chunk is None:
-                return b"data: [DONE]\n\n", True
-            return sse_data(closing_chunk) + b"data: [DONE]\n\n", True
+            if closing_chunk is not None:
+                prefix += sse_data(closing_chunk)
+            if recovery_notice:
+                prefix += sse_data(
+                    recovery_notice_chunk(original_model, recovery_notice)
+                )
+            return prefix + b"data: [DONE]\n\n", True, None
 
         try:
             chunk = json.loads(data.decode("utf-8"))
         except (json.JSONDecodeError, UnicodeDecodeError):
-            return line, False
+            return line, False, recovery_notice
 
         if isinstance(chunk, dict):
+            if recovery_notice and inject_recovery_notice(chunk, recovery_notice):
+                recovery_notice = None
             accumulator.ingest_chunk(chunk)
             stored = accumulator.store_ready_reasoning(self.reasoning_store, scope)
             if stored:
@@ -544,13 +585,17 @@ class DeepSeekProxyHandler(BaseHTTPRequestHandler):
                 chunk["model"] = original_model
             ending = b"\r\n" if line.endswith(b"\r\n") else b"\n"
             return (
+                (
                     b"data: "
-                + json.dumps(chunk, ensure_ascii=False, separators=(",", ":")).encode(
-                    "utf-8"
-                )
+                    + json.dumps(
+                        chunk, ensure_ascii=False, separators=(",", ":")
+                    ).encode("utf-8")
                     + ending
-            ), False
-        return line, False
+                ),
+                False,
+                recovery_notice,
+            )
+        return line, False, recovery_notice
 
 
 def build_arg_parser() -> argparse.ArgumentParser:
@@ -602,10 +647,10 @@ def build_arg_parser() -> argparse.ArgumentParser:
     )
     parser.add_argument(
         "--missing-reasoning-strategy",
-        choices=["reject", "placeholder"],
+        choices=["recover", "reject"],
         help=(
-            "What to do when required reasoning_content is missing: reject "
-            "(safe default) or placeholder (unsafe compatibility fallback)"
+            "What to do when required reasoning_content is missing: "
+            "recover (friendly default) or reject (strict debugging mode)"
         ),
     )
     parser.add_argument(
@@ -740,6 +785,45 @@ def sse_data(payload: dict[str, Any]) -> bytes:
     )
 
 
+def inject_recovery_notice(chunk: dict[str, Any], notice: str) -> bool:
+    choices = chunk.get("choices")
+    if not isinstance(choices, list):
+        return False
+    for choice in choices:
+        if not isinstance(choice, dict):
+            continue
+        delta = choice.get("delta")
+        if not isinstance(delta, dict):
+            continue
+        if "content" not in delta and not delta.get("tool_calls"):
+            continue
+        existing_content = delta.get("content")
+        delta["content"] = notice + (
+            existing_content if isinstance(existing_content, str) else ""
+        )
+        return True
+    return False
+
+
+def recovery_notice_chunk(
+    model: str,
+    notice: str = RECOVERY_NOTICE_CONTENT,
+) -> dict[str, Any]:
+    return {
+        "id": "chatcmpl-deepseek-cursor-proxy-recovery",
+        "object": "chat.completion.chunk",
+        "created": int(time.time()),
+        "model": model,
+        "choices": [
+            {
+                "index": 0,
+                "delta": {"content": notice},
+                "finish_reason": None,
+            }
+        ],
+    }
+
+
 def summarize_chat_payload(payload: dict[str, Any]) -> str:
     messages = payload.get("messages")
     tools = payload.get("tools")
@@ -841,14 +925,6 @@ def main(argv: list[str] | None = None) -> int:
         config.missing_reasoning_strategy,
         config.reasoning_content_path,
     )
-    if config.missing_reasoning_strategy == "placeholder":
-        LOG.warning(
-            (
-                "missing_reasoning_strategy=placeholder is not DeepSeek-compliant; "
-                "use only to test old Cursor histories whose original reasoning "
-                "cannot be recovered"
-            )
-        )
     if config.verbose:
         LOG.info("logging mode=verbose metadata=detailed bodies=true")
         LOG.warning(

src/deepseek_cursor_proxy/transform.py

@@ -67,11 +67,20 @@ CURSOR_THINKING_BLOCK_RE = re.compile(
     re.IGNORECASE,
 )
 
-PLACEHOLDER_REASONING_CONTENT = (
-    "[deepseek-cursor-proxy placeholder reasoning_content: original DeepSeek "
-    "reasoning_content was missing from Cursor history and unavailable in the "
-    "local cache. This is an opt-in compatibility fallback, not the original "
-    "model reasoning.]"
+RECOVERY_NOTICE_TEXT = (
+    "[deepseek-cursor-proxy] Recovered this DeepSeek chat because older "
+    "tool-call reasoning was unavailable; continuing with recent context only."
+)
+LEGACY_RECOVERY_NOTICE_TEXT = (
+    "Note: recovered this DeepSeek chat because older tool-call reasoning "
+    "was unavailable; continuing with recent context only."
+)
+RECOVERY_NOTICE_CONTENT = f"{RECOVERY_NOTICE_TEXT}\n\n"
+RECOVERY_SYSTEM_CONTENT = (
+    "deepseek-cursor-proxy recovered this request because older DeepSeek "
+    "thinking-mode tool-call reasoning_content was unavailable. Older "
+    "unrecoverable tool-call history was omitted; continue using only the "
+    "remaining recovered context."
 )
 
 
@@ -82,8 +91,10 @@ class PreparedRequest:
     upstream_model: str
     cache_namespace: str
     patched_reasoning_messages: int
-    placeholder_reasoning_messages: int
     missing_reasoning_messages: int
+    recovered_reasoning_messages: int = 0
+    recovery_dropped_messages: int = 0
+    recovery_notice: str | None = None
 
 
 def normalize_reasoning_effort(value: Any) -> str:
@@ -203,8 +214,7 @@ def normalize_message(
     cache_namespace: str,
     repair_reasoning: bool,
     keep_reasoning: bool,
-    missing_reasoning_strategy: str,
-) -> tuple[dict[str, Any], bool, bool, bool]:
+) -> tuple[dict[str, Any], bool, bool]:
     if not isinstance(message, dict):
         message = {"role": "user", "content": str(message)}
     normalized = {key: value for key, value in message.items() if key in MESSAGE_FIELDS}
@@ -228,7 +238,6 @@ def normalize_message(
         ]
 
     patched = False
-    placeholder = False
     missing = False
     if normalized["role"] == "assistant":
         if not keep_reasoning:
@@ -249,17 +258,13 @@ def normalize_message(
                         normalized["reasoning_content"] = restored
                         patched = True
                 if needs_reasoning and not patched:
-                    if missing_reasoning_strategy == "placeholder":
-                        normalized["reasoning_content"] = PLACEHOLDER_REASONING_CONTENT
-                        placeholder = True
-                    else:
                     missing = True
 
     allowed_fields = ROLE_MESSAGE_FIELDS.get(str(normalized["role"]), MESSAGE_FIELDS)
     normalized = {
         key: value for key, value in normalized.items() if key in allowed_fields
     }
-    return normalized, patched, placeholder, missing
+    return normalized, patched, missing
 
 
 def normalize_messages(
@@ -268,32 +273,102 @@ def normalize_messages(
     cache_namespace: str,
     repair_reasoning: bool,
     keep_reasoning: bool,
-    missing_reasoning_strategy: str,
-) -> tuple[list[dict[str, Any]], int, int, int]:
+) -> tuple[list[dict[str, Any]], int, list[int]]:
     if not isinstance(messages, list):
-        return [], 0, 0, 0
+        return [], 0, []
     normalized_messages: list[dict[str, Any]] = []
     patched_count = 0
-    placeholder_count = 0
-    missing_count = 0
+    missing_indexes: list[int] = []
     for message in messages:
-        normalized, patched, placeholder, missing = normalize_message(
+        normalized, patched, missing = normalize_message(
             message,
             store,
             normalized_messages,
             cache_namespace,
             repair_reasoning,
             keep_reasoning,
-            missing_reasoning_strategy,
         )
         normalized_messages.append(normalized)
         if patched:
             patched_count += 1
-        if placeholder:
-            placeholder_count += 1
         if missing:
-            missing_count += 1
-    return normalized_messages, patched_count, placeholder_count, missing_count
+            missing_indexes.append(len(normalized_messages) - 1)
+    return normalized_messages, patched_count, missing_indexes
+
+
+def has_recovery_notice(message: dict[str, Any]) -> bool:
+    content = message.get("content")
+    return (
+        message.get("role") == "assistant"
+        and isinstance(content, str)
+        and content.startswith((RECOVERY_NOTICE_TEXT, LEGACY_RECOVERY_NOTICE_TEXT))
+    )
+
+
+def leading_system_messages(messages: list[dict[str, Any]]) -> list[dict[str, Any]]:
+    leading_messages: list[dict[str, Any]] = []
+    for message in messages:
+        if message.get("role") == "system":
+            leading_messages.append(message)
+            continue
+        break
+    return leading_messages
+
+
+def recover_messages_from_missing_reasoning(
+    messages: list[dict[str, Any]],
+    missing_indexes: list[int],
+) -> tuple[list[dict[str, Any]], int, str | None]:
+    recovery_boundary_index = next(
+        (
+            index
+            for index in range(len(messages) - 1, -1, -1)
+            if has_recovery_notice(messages[index])
+            and any(missing_index < index for missing_index in missing_indexes)
+        ),
+        -1,
+    )
+    if recovery_boundary_index != -1:
+        context_user_index = next(
+            (
+                index
+                for index in range(recovery_boundary_index - 1, -1, -1)
+                if messages[index].get("role") == "user"
+            ),
+            -1,
+        )
+        leading_messages = leading_system_messages(messages)
+        recovered_tail = []
+        if context_user_index != -1:
+            recovered_tail.append(messages[context_user_index])
+        recovered_tail.extend(messages[recovery_boundary_index:])
+        recovered = [
+            *leading_messages,
+            {"role": "system", "content": RECOVERY_SYSTEM_CONTENT},
+            *recovered_tail,
+        ]
+        kept_context_messages = 1 if context_user_index != -1 else 0
+        omitted_messages = (
+            recovery_boundary_index - len(leading_messages) - kept_context_messages
+        )
+        return recovered, omitted_messages, None
+
+    last_user_index = next(
+        (
+            index
+            for index in range(len(messages) - 1, -1, -1)
+            if messages[index].get("role") == "user"
+        ),
+        -1,
+    )
+    if last_user_index == -1:
+        return messages, 0, None
+
+    recovered = leading_system_messages(messages)
+    omitted_messages = len(messages) - len(recovered) - 1
+    recovered.append({"role": "system", "content": RECOVERY_SYSTEM_CONTENT})
+    recovered.append(messages[last_user_index])
+    return recovered, omitted_messages, RECOVERY_NOTICE_CONTENT
 
 
 def assistant_needs_reasoning_for_tool_context(
@@ -403,13 +478,32 @@ def prepare_upstream_request(
         prepared.get("reasoning_effort"),
         authorization,
     )
-    messages, patched_count, placeholder_count, missing_count = normalize_messages(
+    messages, patched_count, missing_indexes = normalize_messages(
         payload.get("messages"),
         store,
         cache_namespace,
         repair_reasoning=thinking_enabled,
         keep_reasoning=not thinking_disabled,
-        missing_reasoning_strategy=config.missing_reasoning_strategy,
+    )
+    recovered_count = 0
+    recovery_dropped_messages = 0
+    recovery_notice = None
+    while missing_indexes and config.missing_reasoning_strategy == "recover":
+        recovered_messages, dropped_messages, notice = (
+            recover_messages_from_missing_reasoning(messages, missing_indexes)
+        )
+        if not dropped_messages:
+            break
+        recovered_count += len(missing_indexes)
+        recovery_dropped_messages += dropped_messages
+        if notice:
+            recovery_notice = notice
+        messages, patched_count, missing_indexes = normalize_messages(
+            recovered_messages,
+            store,
+            cache_namespace,
+            repair_reasoning=thinking_enabled,
+            keep_reasoning=not thinking_disabled,
         )
     prepared["messages"] = messages
 
@@ -419,8 +513,10 @@ def prepare_upstream_request(
         upstream_model=upstream_model,
         cache_namespace=cache_namespace,
         patched_reasoning_messages=patched_count,
-        placeholder_reasoning_messages=placeholder_count,
-        missing_reasoning_messages=missing_count,
+        missing_reasoning_messages=len(missing_indexes),
+        recovered_reasoning_messages=recovered_count,
+        recovery_dropped_messages=recovery_dropped_messages,
+        recovery_notice=recovery_notice,
     )
 
 
@@ -452,9 +548,12 @@ def rewrite_response_body(
     store: ReasoningStore | None,
     request_messages: list[dict[str, Any]],
     cache_namespace: str = "",
+    content_prefix: str | None = None,
 ) -> bytes:
     response_payload = json.loads(body.decode("utf-8"))
     if isinstance(response_payload, dict):
+        if content_prefix:
+            prefix_response_content(response_payload, content_prefix)
         record_response_reasoning(
             response_payload, store, request_messages, cache_namespace
         )
@@ -463,3 +562,19 @@ def rewrite_response_body(
     return json.dumps(
         response_payload, ensure_ascii=False, separators=(",", ":")
     ).encode("utf-8")
+
+
+def prefix_response_content(response_payload: dict[str, Any], prefix: str) -> bool:
+    choices = response_payload.get("choices")
+    if not isinstance(choices, list):
+        return False
+    for choice in choices:
+        if not isinstance(choice, dict):
+            continue
+        message = choice.get("message")
+        if not isinstance(message, dict):
+            continue
+        content = message.get("content")
+        message["content"] = prefix + (content if isinstance(content, str) else "")
+        return True
+    return False

tests/test_config.py

@@ -8,6 +8,7 @@ import unittest
 from unittest.mock import patch
 
 from deepseek_cursor_proxy.config import (
+    DEFAULT_CONFIG_TEXT,
     ProxyConfig,
     default_config_path,
     default_reasoning_content_path,
@@ -43,8 +44,35 @@ class ConfigTests(unittest.TestCase):
             self.assertIn(
                 "model: deepseek-v4-pro", config_path.read_text(encoding="utf-8")
             )
+            self.assertNotIn(
+                "missing_reasoning_strategy",
+                config_path.read_text(encoding="utf-8"),
+            )
             self.assertEqual(stat.S_IMODE(config_path.stat().st_mode), 0o600)
             self.assertEqual(config.upstream_model, "deepseek-v4-pro")
+            self.assertEqual(config.missing_reasoning_strategy, "recover")
+
+    def test_legacy_generated_default_config_removes_missing_reasoning_key(
+        self,
+    ) -> None:
+        with TemporaryDirectory() as temp_dir:
+            home = Path(temp_dir)
+
+            with patch("deepseek_cursor_proxy.config.Path.home", return_value=home):
+                config_path = default_config_path()
+                config_path.parent.mkdir(parents=True)
+                config_path.write_text(
+                    DEFAULT_CONFIG_TEXT + "\nmissing_reasoning_strategy: reject\n",
+                    encoding="utf-8",
+                )
+
+                config = ProxyConfig.from_file(env={}, config_path=None)
+
+            self.assertEqual(config.missing_reasoning_strategy, "recover")
+            self.assertNotIn(
+                "missing_reasoning_strategy",
+                config_path.read_text(encoding="utf-8"),
+            )
 
     def test_missing_explicit_config_file_is_not_populated(self) -> None:
         with TemporaryDirectory() as temp_dir:
@@ -76,6 +104,18 @@ class ConfigTests(unittest.TestCase):
         self.assertEqual(config.port, 9100)
         self.assertEqual(config.reasoning_content_path, reasoning_content_path)
 
+    def test_missing_reasoning_strategy_config_key_is_ignored(self) -> None:
+        with TemporaryDirectory() as temp_dir:
+            config_path = Path(temp_dir) / "config.yaml"
+            config_path.write_text(
+                "missing_reasoning_strategy: reject\n",
+                encoding="utf-8",
+            )
+
+            config = ProxyConfig.from_file(env={}, config_path=config_path)
+
+        self.assertEqual(config.missing_reasoning_strategy, "recover")
+
     def test_environment_overrides_config_file(self) -> None:
         with TemporaryDirectory() as temp_dir:
             config_path = Path(temp_dir) / "config.yaml"
@@ -142,7 +182,6 @@ class ConfigTests(unittest.TestCase):
                 "PROXY_NGROK": "yes",
                 "PROXY_CORS": "true",
                 "PROXY_MAX_REQUEST_BODY_BYTES": "1234",
-                "MISSING_REASONING_STRATEGY": "placeholder",
                 "REASONING_CACHE_MAX_AGE_SECONDS": "60",
                 "REASONING_CACHE_MAX_ROWS": "50",
             },
@@ -153,17 +192,17 @@ class ConfigTests(unittest.TestCase):
         self.assertTrue(config.ngrok)
         self.assertTrue(config.cors)
         self.assertEqual(config.max_request_body_bytes, 1234)
-        self.assertEqual(config.missing_reasoning_strategy, "placeholder")
+        self.assertEqual(config.missing_reasoning_strategy, "recover")
         self.assertEqual(config.reasoning_cache_max_age_seconds, 60)
         self.assertEqual(config.reasoning_cache_max_rows, 50)
 
-    def test_invalid_missing_reasoning_strategy_defaults_to_reject(self) -> None:
+    def test_missing_reasoning_strategy_environment_is_ignored(self) -> None:
         config = ProxyConfig.from_file(
-            env={"MISSING_REASONING_STRATEGY": "invent"},
+            env={"MISSING_REASONING_STRATEGY": "reject"},
             config_path=Path("/does/not/exist"),
         )
 
-        self.assertEqual(config.missing_reasoning_strategy, "reject")
+        self.assertEqual(config.missing_reasoning_strategy, "recover")
 
     def test_cursor_reasoning_display_can_be_disabled_from_config(self) -> None:
         with TemporaryDirectory() as temp_dir:

tests/test_proxy_end_to_end.py

@@ -17,7 +17,7 @@ from deepseek_cursor_proxy.reasoning_store import (
 )
 from deepseek_cursor_proxy.server import DeepSeekProxyHandler, DeepSeekProxyServer
 from deepseek_cursor_proxy.transform import (
-    PLACEHOLDER_REASONING_CONTENT,
+    RECOVERY_NOTICE_CONTENT,
     reasoning_cache_namespace,
 )
 
@@ -626,9 +626,14 @@ class ProxyEndToEndTests(unittest.TestCase):
         self.assertIn("too large", payload["error"]["message"])
         self.assertEqual(FakeDeepSeekHandler.requests, [])
 
-    def test_proxy_rejects_uncached_cursor_tool_history_without_placeholder(
+    def test_proxy_rejects_uncached_cursor_tool_history_in_strict_mode(
         self,
     ) -> None:
+        self.proxy.server.config = replace(
+            self.proxy.server.config,
+            missing_reasoning_strategy="reject",
+        )
+
         status, payload = post_json(
             f"{self.proxy.url}/v1/chat/completions",
             second_cursor_request(include_reasoning=False),
@@ -637,29 +642,79 @@ class ProxyEndToEndTests(unittest.TestCase):
         self.assertEqual(status, 409)
         self.assertEqual(payload["error"]["missing_reasoning_messages"], 1)
         self.assertIn("1 assistant message", payload["error"]["message"])
+        self.assertIn("strict missing-reasoning mode", payload["error"]["message"])
         self.assertIn(
-            "not sent upstream",
-            payload["error"]["diagnostic_placeholder"],
+            "--missing-reasoning-strategy recover", payload["error"]["message"]
         )
         self.assertEqual(FakeDeepSeekHandler.requests, [])
 
-    def test_proxy_can_forward_placeholder_for_uncached_cursor_tool_history(
-        self,
-    ) -> None:
-        self.proxy.server.config = replace(
-            self.proxy.server.config,
-            missing_reasoning_strategy="placeholder",
-        )
-
-        status, _ = post_json(
+    def test_proxy_recovers_uncached_cursor_tool_history(self) -> None:
+        with self.assertLogs("deepseek_cursor_proxy", level="WARNING") as captured:
+            status, payload = post_json(
                 f"{self.proxy.url}/v1/chat/completions",
-            second_cursor_request(include_reasoning=False),
+                third_cursor_request_missing_all_reasoning(),
             )
 
         self.assertEqual(status, 200)
+        self.assertTrue(
+            payload["choices"][0]["message"]["content"].startswith(
+                RECOVERY_NOTICE_CONTENT
+            )
+        )
         self.assertEqual(
-            FakeDeepSeekHandler.requests[0]["messages"][1]["reasoning_content"],
-            PLACEHOLDER_REASONING_CONTENT,
+            [
+                message["role"]
+                for message in FakeDeepSeekHandler.requests[0]["messages"]
+            ],
+            ["system", "user"],
+        )
+        self.assertIn(
+            "recovered this request",
+            FakeDeepSeekHandler.requests[0]["messages"][0]["content"],
+        )
+        self.assertEqual(
+            FakeDeepSeekHandler.requests[0]["messages"][1],
+            {"role": "user", "content": "Thanks, now continue."},
+        )
+        self.assertIn(
+            "cached reasoning_content was unavailable",
+            "\n".join(captured.output),
+        )
+
+    def test_proxy_keeps_deepseek_context_after_recovery_boundary(self) -> None:
+        status, first = post_json(
+            f"{self.proxy.url}/v1/chat/completions",
+            third_cursor_request_missing_all_reasoning(),
+        )
+        self.assertEqual(status, 200)
+
+        recovered_assistant = dict(first["choices"][0]["message"])
+        recovered_assistant.pop("reasoning_content", None)
+        payload = third_cursor_request_missing_all_reasoning()
+        payload["messages"].append(recovered_assistant)
+        payload["messages"].append(
+            {
+                "role": "tool",
+                "tool_call_id": "call_date",
+                "content": "2026-04-24",
+            }
+        )
+
+        status, second = post_json(f"{self.proxy.url}/v1/chat/completions", payload)
+
+        self.assertEqual(status, 200)
+        self.assertFalse(
+            second["choices"][0]["message"]["content"].startswith(
+                RECOVERY_NOTICE_CONTENT
+            )
+        )
+        second_upstream_messages = FakeDeepSeekHandler.requests[1]["messages"]
+        self.assertEqual(
+            [message["role"] for message in second_upstream_messages],
+            ["system", "user", "assistant", "tool"],
+        )
+        self.assertEqual(
+            second_upstream_messages[2]["reasoning_content"], TOOL_REASONING
         )
 
 
@@ -893,6 +948,32 @@ class ReasoningStreamingProxyTests(unittest.TestCase):
             "Need context.",
         )
 
+    def test_streaming_recovery_notice_is_visible_in_cursor_content(self) -> None:
+        payload = third_cursor_request_missing_all_reasoning()
+        payload["stream"] = True
+        request = Request(
+            f"{self.proxy.url}/v1/chat/completions",
+            data=json.dumps(payload).encode("utf-8"),
+            method="POST",
+            headers={
+                "Authorization": "Bearer sk-cursor-test",
+                "Content-Type": "application/json",
+            },
+        )
+
+        with urlopen(request, timeout=2) as response:
+            body = response.read().decode("utf-8")
+
+        chunks = [
+            json.loads(line.removeprefix("data: "))
+            for line in body.splitlines()
+            if line.startswith("data: {")
+        ]
+        self.assertEqual(
+            chunks[2]["choices"][0]["delta"]["content"],
+            "\n</think>\n\n" + RECOVERY_NOTICE_CONTENT + FINAL_CONTENT,
+        )
+
 
 class StreamingToolRaceProxyTests(unittest.TestCase):
     def setUp(self) -> None:

tests/test_server.py

@@ -41,6 +41,14 @@ class FakeStreamingResponse:
         return self._lines.pop(0)
 
 
+class FailingStreamingResponse:
+    status = 200
+    headers = {"Content-Type": "text/event-stream"}
+
+    def readline(self) -> bytes:
+        raise OSError("record layer failure")
+
+
 class BrokenPipeWfile:
     def write(self, body: bytes) -> None:
         raise BrokenPipeError("test disconnect")
@@ -158,6 +166,26 @@ class ServerTests(unittest.TestCase):
         self.assertEqual(response.readline_calls, 1)
         self.assertIn("sending streaming response chunk", "\n".join(captured.output))
 
+    def test_streaming_response_handles_upstream_read_failure(self) -> None:
+        handler = make_proxy_handler(BytesIO())
+
+        try:
+            with self.assertLogs("deepseek_cursor_proxy", level="WARNING") as captured:
+                sent = handler._proxy_streaming_response(
+                    FailingStreamingResponse(),
+                    "deepseek-v4-pro",
+                    [{"role": "user", "content": "hi"}],
+                    "cache-namespace",
+                )
+        finally:
+            handler.server.reasoning_store.close()
+
+        self.assertFalse(sent)
+        self.assertIn(
+            "upstream streaming response read failed",
+            "\n".join(captured.output),
+        )
+
 
 if __name__ == "__main__":
     unittest.main()

tests/test_transform.py

@@ -4,9 +4,13 @@ import json
 import unittest
 
 from deepseek_cursor_proxy.config import ProxyConfig
-from deepseek_cursor_proxy.reasoning_store import ReasoningStore, conversation_scope
+from deepseek_cursor_proxy.reasoning_store import (
+    ReasoningStore,
+    conversation_scope,
+    message_signature,
+)
 from deepseek_cursor_proxy.transform import (
-    PLACEHOLDER_REASONING_CONTENT,
+    RECOVERY_NOTICE_CONTENT,
     extract_text_content,
     prepare_upstream_request,
     reasoning_cache_namespace,
@@ -72,7 +76,11 @@ class TransformTests(unittest.TestCase):
             ],
         }
 
-        prepared = prepare_upstream_request(payload, ProxyConfig(), self.store)
+        prepared = prepare_upstream_request(
+            payload,
+            ProxyConfig(missing_reasoning_strategy="reject"),
+            self.store,
+        )
 
         self.assertEqual(prepared.payload["messages"][1]["content"], "Visible answer.")
 
@@ -135,7 +143,11 @@ class TransformTests(unittest.TestCase):
             "tool_choice": "required",
         }
 
-        prepared = prepare_upstream_request(payload, ProxyConfig(), self.store)
+        prepared = prepare_upstream_request(
+            payload,
+            ProxyConfig(missing_reasoning_strategy="reject"),
+            self.store,
+        )
 
         self.assertEqual(prepared.payload["tool_choice"], "required")
 
@@ -201,7 +213,11 @@ class TransformTests(unittest.TestCase):
             ],
         }
 
-        prepared = prepare_upstream_request(payload, ProxyConfig(), self.store)
+        prepared = prepare_upstream_request(
+            payload,
+            ProxyConfig(missing_reasoning_strategy="reject"),
+            self.store,
+        )
 
         self.assertEqual(prepared.patched_reasoning_messages, 1)
         self.assertEqual(
@@ -339,7 +355,11 @@ class TransformTests(unittest.TestCase):
             ],
         }
 
-        prepared = prepare_upstream_request(payload, ProxyConfig(), self.store)
+        prepared = prepare_upstream_request(
+            payload,
+            ProxyConfig(missing_reasoning_strategy="reject"),
+            self.store,
+        )
 
         self.assertEqual(prepared.patched_reasoning_messages, 1)
         self.assertEqual(
@@ -572,16 +592,21 @@ class TransformTests(unittest.TestCase):
             ],
         }
 
-        prepared = prepare_upstream_request(payload, ProxyConfig(), self.store)
+        prepared = prepare_upstream_request(
+            payload,
+            ProxyConfig(missing_reasoning_strategy="reject"),
+            self.store,
+        )
 
         self.assertEqual(prepared.patched_reasoning_messages, 0)
         self.assertEqual(prepared.missing_reasoning_messages, 1)
         self.assertNotIn("reasoning_content", prepared.payload["messages"][1])
 
-    def test_can_insert_placeholder_for_uncached_assistant_tool_call(self) -> None:
+    def test_can_recover_uncached_tool_history_from_latest_user(self) -> None:
         payload = {
             "model": "deepseek-v4-pro",
             "messages": [
+                {"role": "system", "content": "Follow project rules."},
                 {"role": "user", "content": "read README"},
                 {
                     "role": "assistant",
@@ -602,21 +627,227 @@ class TransformTests(unittest.TestCase):
                     "tool_call_id": "call_uncached",
                     "content": "file text",
                 },
+                {"role": "user", "content": "continue with the summary"},
             ],
         }
 
         prepared = prepare_upstream_request(
             payload,
-            ProxyConfig(missing_reasoning_strategy="placeholder"),
+            ProxyConfig(missing_reasoning_strategy="recover"),
             self.store,
         )
 
-        self.assertEqual(prepared.patched_reasoning_messages, 0)
-        self.assertEqual(prepared.placeholder_reasoning_messages, 1)
         self.assertEqual(prepared.missing_reasoning_messages, 0)
+        self.assertEqual(prepared.recovered_reasoning_messages, 1)
+        self.assertEqual(prepared.recovery_dropped_messages, 3)
+        self.assertEqual(prepared.recovery_notice, RECOVERY_NOTICE_CONTENT)
         self.assertEqual(
-            prepared.payload["messages"][1]["reasoning_content"],
-            PLACEHOLDER_REASONING_CONTENT,
+            [message["role"] for message in prepared.payload["messages"]],
+            ["system", "system", "user"],
+        )
+        self.assertIn(
+            "recovered this request", prepared.payload["messages"][1]["content"]
+        )
+        self.assertEqual(
+            prepared.payload["messages"][2],
+            {"role": "user", "content": "continue with the summary"},
+        )
+
+    def test_recovery_boundary_preserves_later_deepseek_tool_context(self) -> None:
+        old_tool_call = {
+            "id": "call_old",
+            "type": "function",
+            "function": {
+                "name": "read_file",
+                "arguments": '{"path":"README.md"}',
+            },
+        }
+        new_tool_call = {
+            "id": "call_new",
+            "type": "function",
+            "function": {
+                "name": "lookup",
+                "arguments": '{"query":"new"}',
+            },
+        }
+        first_recovered = prepare_upstream_request(
+            {
+                "model": "deepseek-v4-pro",
+                "messages": [
+                    {"role": "user", "content": "old model turn"},
+                    {
+                        "role": "assistant",
+                        "content": "",
+                        "tool_calls": [old_tool_call],
+                    },
+                    {
+                        "role": "tool",
+                        "tool_call_id": "call_old",
+                        "content": "old result",
+                    },
+                    {"role": "user", "content": "continue with DeepSeek"},
+                ],
+            },
+            ProxyConfig(missing_reasoning_strategy="recover"),
+            self.store,
+        )
+        recovered_tool_message = {
+            "role": "assistant",
+            "content": RECOVERY_NOTICE_CONTENT,
+            "reasoning_content": "Need the new lookup.",
+            "tool_calls": [new_tool_call],
+        }
+        self.store.store_assistant_message(
+            recovered_tool_message,
+            conversation_scope(
+                first_recovered.payload["messages"],
+                first_recovered.cache_namespace,
+            ),
+        )
+
+        prepared = prepare_upstream_request(
+            {
+                "model": "deepseek-v4-pro",
+                "messages": [
+                    {"role": "user", "content": "old model turn"},
+                    {
+                        "role": "assistant",
+                        "content": "",
+                        "tool_calls": [old_tool_call],
+                    },
+                    {
+                        "role": "tool",
+                        "tool_call_id": "call_old",
+                        "content": "old result",
+                    },
+                    {"role": "user", "content": "continue with DeepSeek"},
+                    {
+                        "role": "assistant",
+                        "content": RECOVERY_NOTICE_CONTENT,
+                        "tool_calls": [new_tool_call],
+                    },
+                    {
+                        "role": "tool",
+                        "tool_call_id": "call_new",
+                        "content": "new result",
+                    },
+                ],
+            },
+            ProxyConfig(missing_reasoning_strategy="recover"),
+            self.store,
+        )
+
+        self.assertEqual(prepared.missing_reasoning_messages, 0)
+        self.assertIsNone(prepared.recovery_notice)
+        self.assertEqual(
+            [message["role"] for message in prepared.payload["messages"]],
+            ["system", "user", "assistant", "tool"],
+        )
+        self.assertEqual(
+            prepared.payload["messages"][2]["reasoning_content"],
+            "Need the new lookup.",
+        )
+        self.assertEqual(
+            prepared.payload["messages"][3],
+            {
+                "role": "tool",
+                "tool_call_id": "call_new",
+                "content": "new result",
+            },
+        )
+
+    def test_recovery_boundary_accepts_legacy_notice_text(self) -> None:
+        legacy_recovery_notice = (
+            "Note: recovered this DeepSeek chat because older tool-call reasoning "
+            "was unavailable; continuing with recent context only.\n\n"
+        )
+        old_tool_call = {
+            "id": "call_old",
+            "type": "function",
+            "function": {
+                "name": "read_file",
+                "arguments": '{"path":"README.md"}',
+            },
+        }
+        new_tool_call = {
+            "id": "call_new",
+            "type": "function",
+            "function": {
+                "name": "lookup",
+                "arguments": '{"query":"new"}',
+            },
+        }
+        first_recovered = prepare_upstream_request(
+            {
+                "model": "deepseek-v4-pro",
+                "messages": [
+                    {"role": "user", "content": "old model turn"},
+                    {
+                        "role": "assistant",
+                        "content": "",
+                        "tool_calls": [old_tool_call],
+                    },
+                    {
+                        "role": "tool",
+                        "tool_call_id": "call_old",
+                        "content": "old result",
+                    },
+                    {"role": "user", "content": "continue with DeepSeek"},
+                ],
+            },
+            ProxyConfig(missing_reasoning_strategy="recover"),
+            self.store,
+        )
+        self.store.store_assistant_message(
+            {
+                "role": "assistant",
+                "content": legacy_recovery_notice,
+                "reasoning_content": "Need the new lookup.",
+                "tool_calls": [new_tool_call],
+            },
+            conversation_scope(
+                first_recovered.payload["messages"],
+                first_recovered.cache_namespace,
+            ),
+        )
+
+        prepared = prepare_upstream_request(
+            {
+                "model": "deepseek-v4-pro",
+                "messages": [
+                    {"role": "user", "content": "old model turn"},
+                    {
+                        "role": "assistant",
+                        "content": "",
+                        "tool_calls": [old_tool_call],
+                    },
+                    {
+                        "role": "tool",
+                        "tool_call_id": "call_old",
+                        "content": "old result",
+                    },
+                    {"role": "user", "content": "continue with DeepSeek"},
+                    {
+                        "role": "assistant",
+                        "content": legacy_recovery_notice,
+                        "tool_calls": [new_tool_call],
+                    },
+                    {
+                        "role": "tool",
+                        "tool_call_id": "call_new",
+                        "content": "new result",
+                    },
+                ],
+            },
+            ProxyConfig(missing_reasoning_strategy="recover"),
+            self.store,
+        )
+
+        self.assertEqual(prepared.missing_reasoning_messages, 0)
+        self.assertIsNone(prepared.recovery_notice)
+        self.assertEqual(
+            prepared.payload["messages"][2]["reasoning_content"],
+            "Need the new lookup.",
         )
 
     def test_reports_missing_reasoning_for_uncached_assistant_after_tool_result(
@@ -651,7 +882,11 @@ class TransformTests(unittest.TestCase):
             ],
         }
 
-        prepared = prepare_upstream_request(payload, ProxyConfig(), self.store)
+        prepared = prepare_upstream_request(
+            payload,
+            ProxyConfig(missing_reasoning_strategy="reject"),
+            self.store,
+        )
 
         self.assertEqual(prepared.missing_reasoning_messages, 1)
         self.assertNotIn("reasoning_content", prepared.payload["messages"][3])
@@ -707,7 +942,7 @@ class TransformTests(unittest.TestCase):
         self.assertNotIn("reasoning_content", prepared.payload["messages"][1])
 
     def test_reasoning_cache_is_namespaced_by_authorization(self) -> None:
-        config = ProxyConfig()
+        config = ProxyConfig(missing_reasoning_strategy="reject")
         prior = [{"role": "user", "content": "read README"}]
         namespace_a = reasoning_cache_namespace(
             config,
@@ -814,6 +1049,58 @@ class TransformTests(unittest.TestCase):
             "I need to inspect the repo.",
         )
 
+    def test_rewrite_response_can_prefix_recovery_notice_before_storing(
+        self,
+    ) -> None:
+        body = json.dumps(
+            {
+                "id": "chatcmpl-test",
+                "object": "chat.completion",
+                "model": "deepseek-v4-pro",
+                "choices": [
+                    {
+                        "index": 0,
+                        "finish_reason": "stop",
+                        "message": {
+                            "role": "assistant",
+                            "content": "Summary.",
+                            "reasoning_content": "Tool result is enough.",
+                        },
+                    }
+                ],
+            }
+        ).encode()
+
+        request_messages = [
+            {"role": "user", "content": "read README"},
+            {"role": "tool", "tool_call_id": "call_abc", "content": "file text"},
+        ]
+        rewritten = rewrite_response_body(
+            body,
+            "deepseek-v4-pro",
+            self.store,
+            request_messages,
+            content_prefix=RECOVERY_NOTICE_CONTENT,
+        )
+        payload = json.loads(rewritten)
+        stored_message = {
+            "role": "assistant",
+            "content": RECOVERY_NOTICE_CONTENT + "Summary.",
+            "reasoning_content": "Tool result is enough.",
+        }
+
+        self.assertEqual(
+            payload["choices"][0]["message"]["content"],
+            RECOVERY_NOTICE_CONTENT + "Summary.",
+        )
+        self.assertEqual(
+            self.store.get(
+                f"scope:{conversation_scope(request_messages)}:signature:"
+                f"{message_signature(stored_message)}"
+            ),
+            "Tool result is enough.",
+        )
+
     def test_rewrite_response_preserves_prompt_cache_usage_fields(self) -> None:
         body = json.dumps(
             {