security(phase-3): sanitize error responses to MCP clients (M3)#20
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Add src/mcp/errors.ts with `sanitizeForClient` and `stripAbsolutePaths`.
Route every `catch (err) { ...return { error: String(err) } }` site
through the helper so caught exceptions can't leak absolute filesystem
paths (which embed the user's home dir, repo path, and often customer
or project names) to MCP / HTTP clients.
The helper:
- Logs the full original error to stderr (operator-visible).
- Returns the message with absolute paths (POSIX, Windows drive,
UNC) replaced by `<path>`.
Wired into:
- src/mcp/http.ts:113 — /mcp 500-handler
- src/mcp/tools.ts — every tool's catch block (search_docs,
list_docs, reindex_docs, get, multi_get, set_context,
list_contexts, remove_context), plus the per-file error inside
multi_get's loop.
Note: indexer.resolveRef error sites were already addressed in
Phase 1's refactor (they now return relFile-based errors that
never embed workspaceRoot). This phase covers the catch-handler
escape hatch and the OpenAI passthrough flagged in M1's notes.
Tests
- mcp-errors.test.ts: 9 cases for stripAbsolutePaths (POSIX,
Windows drive, UNC, multiple paths, path-only message,
relative paths preserved) and sanitizeForClient (stderr log,
non-Error throwables, context-hint prefix).
- mcp-tools.test.ts: 3 regression tests that throw an Error
embedding an absolute path from search_docs, reindex_docs, and
get; assert the client response contains the error context but
not `/Users/`, `secret-client`, or `/var/log`.
349 tests pass (was 337; added 12).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Phase 3 of plans/security-fixes-2026-05.md.
Summary
Add a tiny error-sanitization layer (`src/mcp/errors.ts`) and route every catch-handler in the MCP transport / tool surface through it. The helper logs the full error to stderr for the operator and returns the message with absolute paths (POSIX, Windows drive, UNC) replaced by ``.
Phase 1 already removed `absPath` interpolation from `indexer.resolveRef` error returns; this phase closes the catch-handler escape hatch.
Test plan
🤖 Generated with Claude Code