fix: schema compaction must not drop property names that match DROP_KEYS (#785)

Fixes #759

## Summary

`_compact_openai_tool_schema_value()` strips every key matching
`_OPENAI_TOOL_SCHEMA_DROP_KEYS` (which includes `title`, `readOnly`,
`deprecated`, `writeOnly`, etc.) regardless of where in the schema tree
it appears. This is wrong when those same strings are used as **property
names** inside a `properties` object — they're valid business fields,
not annotation metadata.

The result is an invalid strict schema sent upstream:
```
"required key 'title' not in properties"
```

**Root cause (single function, two lines):**
```python
# before — drops "title" everywhere, even as a property name
for key, child in value.items():
    if key in _OPENAI_TOOL_SCHEMA_DROP_KEYS:
        continue
    compacted[key] = _compact_openai_tool_schema_value(child)
```

**Fix — add `_parent_key` context, skip drop only when not inside
`properties`:**
```python
def _compact_openai_tool_schema_value(value, _parent_key=None):
    ...
    for key, child in value.items():
        if _parent_key != "properties" and key in _OPENAI_TOOL_SCHEMA_DROP_KEYS:
            continue
        compacted[key] = _compact_openai_tool_schema_value(child, key)
```

Schema-level annotations (e.g. `title: "ReadFileParameters"` at schema
root) are **still stripped**. Only property names whose string value
happens to match a drop-key are preserved.

## Test plan

- [x] Added
`test_openai_tool_schema_compaction_preserves_property_named_title` in
`tests/test_openai_responses_context_compaction.py` — reproduces the
exact OMP `eval` tool schema from the issue report
- [x] All 9 existing compaction tests still pass (including
`test_openai_tool_schema_compaction_preserves_invocation_shape` which
verifies schema-level `title` is still stripped)

```
tests/test_openai_responses_context_compaction.py::test_openai_tool_schema_compaction_preserves_invocation_shape PASSED
tests/test_openai_responses_context_compaction.py::test_openai_tool_schema_compaction_preserves_property_named_title PASSED
tests/test_openai_responses_context_compaction.py::test_openai_tool_schema_compaction_is_deterministic PASSED
9 passed
```

## Real behavior proof

- **OS**: macOS darwin arm64, Python 3.11.0
- **Tested**: ran the new and existing compaction tests locally against
the patched handler
- **Not tested**: live OMP / Venice.ai / Codex endpoint (no API key for
those); the fix is a pure schema-transform function with no network side
effects

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

headroom/proxy/handlers/openai.py

@@ -215,23 +215,27 @@ def _json_byte_len(value: Any) -> int:
 
 def _compact_openai_tool_schema_value(
     value: Any,
+    _parent_key: str | None = None,
 ) -> Any:
     if isinstance(value, list):
-        return [_compact_openai_tool_schema_value(item) for item in value]
+        return [_compact_openai_tool_schema_value(item, _parent_key) for item in value]
 
     if not isinstance(value, dict):
         return value
 
     compacted: dict[str, Any] = {}
     for key, child in value.items():
-        if key in _OPENAI_TOOL_SCHEMA_DROP_KEYS:
+        # Don't drop keys that are property *names* inside a JSON Schema
+        # `properties` object — only drop them when they are schema annotations.
+        # e.g. a tool with a field literally named "title" must not be stripped.
+        if _parent_key != "properties" and key in _OPENAI_TOOL_SCHEMA_DROP_KEYS:
             continue
 
         if key == "description" and isinstance(child, str):
             compacted[key] = " ".join(child.split())
             continue
 
-        compacted[key] = _compact_openai_tool_schema_value(child)
+        compacted[key] = _compact_openai_tool_schema_value(child, key)
 
     return compacted
 

tests/test_openai_responses_context_compaction.py

@@ -99,6 +99,66 @@ def test_openai_tool_schema_compaction_preserves_invocation_shape() -> None:
     assert tool["parameters"]["properties"]["path"]["description"] == " ".join(verbose.split())
 
 
+def test_openai_tool_schema_compaction_preserves_property_named_title() -> None:
+    """Issue #759: drop-key list must not strip property *names* under `properties`.
+
+    Schema annotations like ``title: "ReadFileParameters"`` on a schema object
+    are safe to drop.  But a tool that has a field literally called ``title``
+    (or ``readOnly``, ``deprecated``, etc.) must survive compaction; removing
+    it while leaving ``required: ["title"]`` produces an invalid strict schema
+    that upstream (OpenAI / Codex) rejects.
+    """
+    payload = {
+        "tools": [
+            {
+                "type": "function",
+                "name": "eval",
+                "description": "Evaluate cells.",
+                "parameters": {
+                    "$schema": "https://json-schema.org/draft/2020-12/schema",
+                    "title": "EvalParameters",
+                    "type": "object",
+                    "properties": {
+                        "cells": {
+                            "type": "array",
+                            "items": {
+                                "type": "object",
+                                "title": "CellItem",
+                                "properties": {
+                                    "language": {"type": "string"},
+                                    "code": {"type": "string"},
+                                    "title": {"type": "string"},
+                                },
+                                "required": ["language", "code", "title"],
+                            },
+                        }
+                    },
+                    "required": ["cells"],
+                },
+            }
+        ]
+    }
+
+    compacted, modified, before, after = _compact_openai_responses_tools(payload)
+
+    assert modified is True
+    assert after < before
+
+    params = compacted["tools"][0]["parameters"]
+    # Schema-level annotations are still dropped.
+    assert "title" not in params
+    assert "$schema" not in params
+
+    items = params["properties"]["cells"]["items"]
+    # "title" as a JSON Schema annotation on the items object is dropped.
+    assert "title" not in items
+    # "title" as a *property name* inside properties must be preserved.
+    assert "title" in items["properties"], (
+        "property named 'title' was incorrectly stripped by compaction"
+    )
+    assert items["required"] == ["language", "code", "title"]
+
+
 def test_openai_tool_schema_compaction_is_deterministic() -> None:
     payload = {
         "tools": [