fix(router): honor MCP aliases in excluded tools (#1822) (#1863)

## Description

Normalize MCP tool-name aliases in the shared exclusion matcher so
Anthropic/custom-agent names like `mcp_Server_tool` match the documented
`mcp__*` glob and bare tool exclusions such as `headroom_retrieve`.

Closes #1822

## Type of Change

- [x] Bug fix (non-breaking change that fixes an issue)
- [ ] New feature (non-breaking change that adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Documentation update
- [ ] Performance improvement
- [ ] Code refactoring (no functional changes)

## Changes Made

- Added MCP alias matching for `mcp__server__tool`, `mcp_Server_tool`,
and the bare wrapped tool name.
- Added Anthropic `tool_use` / `tool_result` regressions for
custom-agent MCP names and bare `headroom_retrieve` exclusions.

## Testing

- [x] Unit tests pass (`pytest`)
- [x] Linting passes (`ruff check .`)
- [x] Type checking passes (`mypy headroom`)
- [x] New tests added for new functionality
- [x] Manual testing performed

### Test Output

```text
$ .venv/bin/python -m pytest tests/test_transforms/test_content_router.py -q
57 passed, 1 warning in 0.95s

$ .venv/bin/python -m ruff check .
All checks passed!

$ .venv/bin/python -m ruff format --check .
1058 files already formatted

$ .venv/bin/python -m mypy headroom --ignore-missing-imports
Success: no issues found in 407 source files
```

## Real Behavior Proof

- Environment: macOS, Python 3.13.5 local venv with editable headroom
build.
- Exact command / steps: Added #1822 regressions, ran the focused tests
before the fix, then reran after adding MCP aliases.
- Observed result: Before the fix, custom-agent MCP tool results were
compressed instead of excluded; after the fix, the full content-router
test file passes and excluded MCP results stay on the lossless excluded
path.
- Not tested: Full repository test suite locally; GitHub CI passed the
full PR matrix.

## Review Readiness

- [x] I have performed a self-review
- [x] This PR is ready for human review
- [x] Principal engineer agent approved
- [x] Senior developer agent approved

## Checklist

- [x] My code follows the project style guidelines
- [x] I have performed a self-review of my code
- [x] I have commented my code, particularly in hard-to-understand areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] I have updated the CHANGELOG.md if applicable

## Screenshots (if applicable)

N/A

## Additional Notes

Review agents approved the scoped MCP exclusion-alias fix. One
non-blocking review note: #1822 also mentions TOIN/prefix-cache
symptoms, while this PR specifically fixes the custom-agent MCP
exclusion name-resolution path.

Author: aivinay

headroom/config.py

@@ -229,6 +229,27 @@ class AnchorConfig:
 )
 
 
+def _tool_name_aliases(name: str) -> tuple[str, ...]:
+    """Return equivalent spellings for tool exclusion matching."""
+    aliases = [name]
+    lname = name.lower()
+
+    if lname.startswith("mcp__"):
+        # OpenAI-style MCP wrappers use mcp__server__tool. Custom agents that
+        # speak Anthropic sometimes emit the same wrapper as mcp_Server_tool.
+        parts = name.split("__", 2)
+        if len(parts) == 3 and parts[1] and parts[2]:
+            aliases.append(f"mcp_{parts[1]}_{parts[2]}")
+            aliases.append(parts[2])
+    elif lname.startswith("mcp_"):
+        parts = name.split("_", 2)
+        if len(parts) == 3 and parts[1] and parts[2]:
+            aliases.append(f"mcp__{parts[1]}__{parts[2]}")
+            aliases.append(parts[2])
+
+    return tuple(dict.fromkeys(aliases))
+
+
 def is_tool_excluded(name: str, exclude_tools: Iterable[str]) -> bool:
     """Return True if ``name`` matches the tool-exclusion set.
 
@@ -237,15 +258,28 @@ def is_tool_excluded(name: str, exclude_tools: Iterable[str]) -> bool:
     ``[``) are matched with :func:`fnmatch.fnmatchcase`, letting a single pattern
     such as ``mcp__*`` cover every tool an MCP server exposes without listing
     each name (issue #870).
+
+    MCP tool wrappers are also matched through their common aliases. For example,
+    ``mcp__Headroom__headroom_retrieve`` and
+    ``mcp_Headroom_headroom_retrieve`` both match ``mcp__*`` and the bare
+    ``headroom_retrieve`` entry.
     """
     if not exclude_tools:
         return False
-    if name in exclude_tools or name.lower() in exclude_tools:
+
+    patterns = tuple(exclude_tools)
+    if not patterns:
+        return False
+    aliases = _tool_name_aliases(name)
+    exact_patterns = set(patterns)
+    lower_exact_patterns = {pat.lower() for pat in exact_patterns}
+    if any(alias in exact_patterns or alias.lower() in lower_exact_patterns for alias in aliases):
         return True
-    lname = name.lower()
+
     return any(
-        fnmatch.fnmatchcase(lname, pat.lower())
-        for pat in exclude_tools
+        fnmatch.fnmatchcase(alias.lower(), pat.lower())
+        for alias in aliases
+        for pat in patterns
         if "*" in pat or "?" in pat or "[" in pat
     )
 

tests/test_transforms/test_content_router.py

@@ -796,16 +796,100 @@ def test_glob_exclude_tools(self, tokenizer):
         assert json.loads(result.messages[1]["content"]) == json.loads(messages[1]["content"])
         assert "router:excluded:lossless_json" in result.transforms_applied
 
+    def test_anthropic_mcp_alias_exclude_tools(self, tokenizer):
+        """Single-underscore MCP names from custom agents honor documented MCP globs."""
+        config = ContentRouterConfig(
+            min_section_tokens=10,
+            exclude_tools={"mcp__*"},
+        )
+        router = ContentRouter(config)
+
+        messages = [
+            {
+                "role": "assistant",
+                "content": [
+                    {
+                        "type": "tool_use",
+                        "id": "toolu_mcp_1",
+                        "name": "mcp_CursorTaskRegistry_cursor_list_tasks",
+                        "input": {"project": "headroom"},
+                    }
+                ],
+            },
+            {
+                "role": "user",
+                "content": [
+                    {
+                        "type": "tool_result",
+                        "tool_use_id": "toolu_mcp_1",
+                        "content": generate_json_data(50),
+                    }
+                ],
+            },
+        ]
+
+        result = router.apply(messages, tokenizer)
+
+        tool_result_block = result.messages[1]["content"][0]
+        assert json.loads(tool_result_block["content"]) == json.loads(
+            messages[1]["content"][0]["content"]
+        )
+        assert "router:excluded:lossless_json" in result.transforms_applied
+
+    def test_anthropic_mcp_bare_tool_alias_exclude_tools(self, tokenizer):
+        """Bare tool exclusions match custom-agent MCP wrappers (#1822)."""
+        config = ContentRouterConfig(
+            min_section_tokens=10,
+            exclude_tools={"headroom_retrieve"},
+        )
+        router = ContentRouter(config)
+
+        messages = [
+            {
+                "role": "assistant",
+                "content": [
+                    {
+                        "type": "tool_use",
+                        "id": "toolu_retrieve_1",
+                        "name": "mcp_HeadroomZai_headroom_retrieve",
+                        "input": {"key": "abc123"},
+                    }
+                ],
+            },
+            {
+                "role": "user",
+                "content": [
+                    {
+                        "type": "tool_result",
+                        "tool_use_id": "toolu_retrieve_1",
+                        "content": generate_json_data(50),
+                    }
+                ],
+            },
+        ]
+
+        result = router.apply(messages, tokenizer)
+
+        tool_result_block = result.messages[1]["content"][0]
+        assert json.loads(tool_result_block["content"]) == json.loads(
+            messages[1]["content"][0]["content"]
+        )
+        assert "router:excluded:lossless_json" in result.transforms_applied
+
     def test_is_tool_excluded_helper(self):
         """is_tool_excluded: exact (case-insensitive) and glob matching."""
         from headroom.config import is_tool_excluded
 
         # Glob entry covers a whole MCP server; unrelated tools are untouched.
         assert is_tool_excluded("mcp__build123d__measure", {"mcp__*"})
+        assert is_tool_excluded("mcp_CursorTaskRegistry_cursor_list_tasks", {"mcp__*"})
         assert not is_tool_excluded("Bash", {"mcp__*"})
         # Plain entries keep exact, case-insensitive membership.
         assert is_tool_excluded("Read", {"read"})
         assert is_tool_excluded("MCP__X", {"mcp__*"})
+        # MCP wrapper aliases can still be excluded by their bare tool name.
+        assert is_tool_excluded("mcp_HeadroomZai_headroom_retrieve", {"headroom_retrieve"})
+        assert is_tool_excluded("mcp__Headroom__headroom_retrieve", {"headroom_retrieve"})
         # Empty set never excludes.
         assert not is_tool_excluded("Read", set())