fix(context_compressor): keep tool-call arguments JSON valid when shrinking

Pass 3 of `_prune_old_tool_results` previously shrunk long `function.arguments`
blobs by slicing the raw JSON string at byte 200 and appending the literal
text `...[truncated]`. That routinely produced payloads like::

    {"path": "/foo.md", "content": "# Long markdown
    ...[truncated]

— an unterminated string with no closing brace. Strict providers (observed
on MiniMax) reject this as `invalid function arguments json string` with a
non-retryable 400. Because the broken call survives in the session history,
every subsequent turn re-sends the same malformed payload and gets the same
400, locking the session into a re-send loop until the call falls out of
the window.

Fix: parse the arguments first, shrink long string leaves inside the parsed
structure, and re-serialise. Non-string values (paths, ints, booleans, lists)
pass through intact. Arguments that are not valid JSON to begin with (rare,
some backends use non-JSON tool args) are returned unchanged rather than
replaced with something neither we nor the provider can parse.

Observed in the wild: a `write_file` with ~800 chars of markdown `content`
triggered this on a real session against MiniMax-M2.7; every turn after
compression got rejected until the session was manually reset.

Tests:
- 7 direct tests of `_truncate_tool_call_args_json` covering valid-JSON
  output, non-JSON pass-through, nested structures, non-string leaves,
  scalar JSON, and Unicode preservation
- 1 end-to-end test through `_prune_old_tool_results` Pass 3 that
  reproduces the exact failure payload shape from the incident

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

agent/context_compressor.py

@@ -63,6 +63,52 @@ _CHARS_PER_TOKEN = 4
 _SUMMARY_FAILURE_COOLDOWN_SECONDS = 600
 
 
+def _truncate_tool_call_args_json(args: str, head_chars: int = 200) -> str:
+    """Shrink long string values inside a tool-call arguments JSON blob while
+    preserving JSON validity.
+
+    The ``function.arguments`` field on a tool call is a JSON-encoded string
+    passed through to the LLM provider; downstream providers strictly
+    validate it and return a non-retryable 400 when it is not well-formed.
+    An earlier implementation sliced the raw JSON at a fixed byte offset and
+    appended ``...[truncated]`` — which routinely produced strings like::
+
+        {"path": "/foo/bar", "content": "# long markdown
+        ...[truncated]
+
+    i.e. an unterminated string and a missing closing brace. MiniMax, for
+    example, rejects this with ``invalid function arguments json string``
+    and the session gets stuck re-sending the same broken history on every
+    turn. See issue #11762 for the observed loop.
+
+    This helper parses the arguments, shrinks long string leaves inside the
+    parsed structure, and re-serialises. Non-string values (paths, ints,
+    booleans) are preserved intact. If the arguments are not valid JSON
+    to begin with — some model backends use non-JSON tool arguments — the
+    original string is returned unchanged rather than replaced with
+    something neither we nor the backend can parse.
+    """
+    try:
+        parsed = json.loads(args)
+    except (ValueError, TypeError):
+        return args
+
+    def _shrink(obj: Any) -> Any:
+        if isinstance(obj, str):
+            if len(obj) > head_chars:
+                return obj[:head_chars] + "...[truncated]"
+            return obj
+        if isinstance(obj, dict):
+            return {k: _shrink(v) for k, v in obj.items()}
+        if isinstance(obj, list):
+            return [_shrink(v) for v in obj]
+        return obj
+
+    shrunken = _shrink(parsed)
+    # ensure_ascii=False preserves CJK/emoji instead of bloating with \uXXXX
+    return json.dumps(shrunken, ensure_ascii=False)
+
+
 def _summarize_tool_result(tool_name: str, tool_args: str, tool_content: str) -> str:
     """Create an informative 1-line summary of a tool call + result.
 
@@ -449,6 +495,11 @@ class ContextCompressor(ContextEngine):
         # Pass 3: Truncate large tool_call arguments in assistant messages
         # outside the protected tail. write_file with 50KB content, for
         # example, survives pruning entirely without this.
+        #
+        # The shrinking is done inside the parsed JSON structure so the
+        # result remains valid JSON — otherwise downstream providers 400
+        # on every subsequent turn until the broken call falls out of
+        # the window. See ``_truncate_tool_call_args_json`` docstring.
         for i in range(prune_boundary):
             msg = result[i]
             if msg.get("role") != "assistant" or not msg.get("tool_calls"):
@@ -459,8 +510,10 @@ class ContextCompressor(ContextEngine):
                 if isinstance(tc, dict):
                     args = tc.get("function", {}).get("arguments", "")
                     if len(args) > 500:
-                        tc = {**tc, "function": {**tc["function"], "arguments": args[:200] + "...[truncated]"}}
-                        modified = True
+                        new_args = _truncate_tool_call_args_json(args)
+                        if new_args != args:
+                            tc = {**tc, "function": {**tc["function"], "arguments": new_args}}
+                            modified = True
                 new_tcs.append(tc)
             if modified:
                 result[i] = {**msg, "tool_calls": new_tcs}