fix(cursor): fail closed on malformed tool arguments - #1680
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📝 WalkthroughWalkthroughCursor event processing now carries request-declared freeform tool names, preserves incomplete streamed arguments, validates completed freeform payloads, and finalizes terminated calls. Bridge processing validates arguments and stops after batch errors. Tests cover these paths. ChangesTool-call argument handling
Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: ⚪ Minimal · up to The PR makes malformed tool calls fail closed while preserving valid delayed native arguments and preventing later calls from appearing successful; focused validation passes, and no actionable merge-blocking risk remains at the current head. Possibly related PRs
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Inline comments:
In `@src/bridge.ts`:
- Around line 94-100: The toolCallArgumentsUsable function should retain trimmed
input only for the whitespace-only check, then pass the original args string to
JSON.parse so JSON-invalid U+00A0 and U+FEFF prefixes are rejected. Add a
regression test verifying "\u00A0{}" produces an incomplete call and failed
response.
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Inline comments:
In `@src/adapters/cursor/protobuf-events.ts`:
- Around line 577-579: Update the toolCallCompleted handling near commitToolCall
so an empty completed freeform call commits its buffered arguments through
commitToolCall, allowing cursorFreeformWrapperValid and dropInvalidFreeformCall
to reject and close it. Retain the deferred completion path only for
non-freeform calls that may receive native arguments later, and add a regression
test covering toolCallStarted followed by an empty toolCallCompleted.
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In `@src/adapters/cursor/protobuf-events.ts`:
- Around line 700-704: Update the invalid-wrapper handling around
cursorFreeformWrapperValid so only empty or syntactically incomplete buffered
arguments are deferred; allow complete JSON such as {} and {"input":1} to reach
commitToolCall for closed wrapper validation and error emission. Add regression
coverage for compact completion after each buffered payload.
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[Repository bug audit · 2026-08-14]
No code-level blocker found in the reviewed diff. Preserving malformed buffered arguments, binding freeform handling to the request-local advertised catalog, validating the {input:string} wrapper, and stopping non-streaming completion after the first malformed call form a coherent fail-closed contract. The focused regressions cover the important streamed/native-late-argument cases.
Merge condition: approve/run the currently action_required Cross-platform CI and React Doctor workflows on this exact head. If they pass, this is a high-priority Cursor correctness/security fix and can merge without waiting for the broader Cursor refactors.
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Inline comments:
In `@tests/cursor-protobuf-events.test.ts`:
- Around line 474-521: Add a regression test alongside the existing
partial-wrapper cases for a freeform tool call whose buffered arguments are
complete JSON but have an invalid shape, such as {"input":1}. Verify completion
defers emission while the call remains in openToolCalls, then verify
turnEndedFrame produces an error containing the call ID and clears the pending
call when no native arguments arrive.
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Cherry-picked onto dev as part of the bug resolution campaign (commit-and-merge loop). Changes verified with typecheck and focused tests. |
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Summary
toolCallCompletedomits the authoritative argument map.{ input: string }wrapper before emitting executable tool events.Root cause and impact
Cursor can stream cumulative tool-call argument text and then complete without a protobuf argument map. The adapter previously replaced incomplete or malformed buffered JSON with an empty string, which the bridge treated as a legitimate no-argument call and serialized as
{}.Preserving the buffer lets ordinary calls fail closed, but request-declared freeform tools intentionally bypass the bridge's ordinary JSON check. Cursor now carries request-local freeform provenance and validates its advertised wrapper before commit. A completion with no argument bytes waits when that wrapper is absent or invalid, including when the call first appears only in the completion frame. A later same-ID native
mcpArgsframe can then finish a legitimate call. If it never arrives, turn finalization rejects the still-open call, closes its translator-budget lease, and prevents post-terminal frames from reopening it.The waiting transition emits one liveness heartbeat, while repeated completion frames for the same pending call do not keep refreshing the watchdog. Client tool-call records are capped per turn, so unique completion-only frames cannot grow retained request state without bound.
The shared bridge predicate also parses the original bytes after its whitespace-only check. JavaScript
trim()removes NBSP and BOM even though JSON does not accept them as whitespace. The non-streaming path now also stops after the first failure so later parallel calls cannot appear completed inside a failed response.Verification
devat36aed0bf0e34db035e724dbe6b2509972c29cbbf; exact head:2d84e08c263562a9300d90c16a55e7f4f7945f01.bun run typecheckpassed on both runtimes.bun run privacy:scanandgit diff --check origin/dev...HEADpassed.db57300e-7426-4f82-86ea-72b3dbb540adcovered all six changed files at36aed0bf...2d84e08c, reported complete coverage, and found no reportable issue.Scope notes
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Bug Fixes
input.Tests