fix(tool): preserve remote MCP error diagnostics

[kojiwakayama]

Summary

• Preserve actionable MCP text when structuredContent is empty.
• Keep stable bracketed and outer error codes, plus the current tool-call correlation ID.
• Return tools/call JSON-RPC errors as bounded structured failures while keeping discovery and transport failures on the existing exception path.
• Verify structured failures survive durable replay and remain displayable through getToolResultError.

Red-green TDD

• RED: an isError response with empty structuredContent returned only an opaque error marker instead of its valid text diagnostic.
• GREEN: the adapter falls back to text, preserves the stable code and correlation ID, and exposes the actionable message.
• RED: a top-level JSON-RPC tools/call error threw and discarded its stable code and request correlation data.
• GREEN: tool execution returns a bounded structured failure. JSON-RPC discovery errors and HTTP transport failures still throw.

Verification

• deno test for src/tool plus the related MCP source, runtime continuation, tracing, and durable replay tests: 83 suites, 239 steps passed.
• deno check src/tool/remote-mcp.ts: passed.
• deno fmt and git diff --check: passed.
• The repository-wide unit command stops before test execution because scripts/build/dnt-meta-property-safety.ts imports the unmapped #babel/parser specifier. This reproduces on the untouched base commit.

Studio

No Studio UI code changes are required. The existing errorText path receives the preserved message through getToolResultError, and the durable replay regression covers the structured payload.

Closes veryfront/veryfront-issue-inbox#10

Summary by CodeRabbit

• Bug Fixes
  • Remote tool errors now appear as structured, model-readable results with clearer error codes and diagnostic details.
  • JSON-RPC tool errors preserve relevant request IDs, nested codes, and numeric protocol codes where available.
  • Correlation IDs are validated and bounded for more reliable error tracking.
  • Unexpected HTTP failures continue to surface as exceptions rather than being treated as normal tool results.
  • Provider-executed tool error results now retain structured diagnostic information.

[github-actions]

📦 Client bundle boundary

Entrypoint	Modules	Source size	Server leaks
src/index.client.ts	455	3071 KiB	⚠️ 39 known

A server module in a client graph aborts hydration in the browser. New leaks fail CI; known leaks are tracked in scripts/lint/client-bundle-baseline.json to burn down.

[coderabbitai]

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Review details

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro Plus

Run ID: d34971c0-8669-478b-b7da-ad7e2d09391f

📥 Commits

Reviewing files that changed from the base of the PR and between 249fce9 and d50b1a1.

📒 Files selected for processing (3)

• docs/api-reference/veryfront/tool.md
• src/tool/remote-mcp.test.ts
• src/tool/remote-mcp.ts

📝 Walkthrough

Walkthrough

Changes

The PR adds structured normalization for remote MCP text and JSON-RPC tool errors. Tool-call errors can remain model-visible results with bounded codes, messages, request IDs, and correlation IDs. Non-tool protocol failures remain exceptions. Tests cover provider round trips and HTTP failures.

MCP tool error handling

Layer / File(s)	Summary
Error payload parsing and validation src/tool/remote-mcp.ts	The remote MCP adapter validates bounded correlation IDs and builds structured payloads from text and JSON-RPC errors.
Preserved error execution flow src/tool/remote-mcp.ts	Tool-call JSON-RPC errors return marked results for normalization. Other protocol failures continue through exception handling.
Error round-trip and regression coverage src/tool/remote-mcp.test.ts, src/agent/conversation/legacy-run-read-adapter.test.ts	Tests cover structured diagnostics, remote error codes, JSON-RPC metadata, correlation IDs, preserved provider output, and unexpected HTTP failures.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🔵 Low · up to 249fc

The change preserves useful MCP diagnostics, but an unusually long tool-call ID could make stored or displayed error results unnecessarily large. The PR is mergeable with owner awareness and a bounded-ID follow-up.

Sequence Diagram(s)

sequenceDiagram
  participant RemoteMCP
  participant ToolExecution
  participant ErrorNormalizer
  participant Provider
  RemoteMCP->>ToolExecution: return marked structured tool error
  ToolExecution->>ErrorNormalizer: normalize error details
  ErrorNormalizer-->>Provider: provide bounded model-visible result
  Provider-->>ToolExecution: preserve structured diagnostic

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Possibly related PRs

• veryfront/veryfront-code#3429: Related provider-executed tool finalization and terminal error-result handling.

Suggested reviewers: kwakayama

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: preserving diagnostics for remote MCP tool errors.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-10-preserve-mcp-errors

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[chatgpt-codex-connector]

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Actionable comments posted: 1

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Inline comments:
In `@src/tool/remote-mcp.ts`:
- Line 311: Update the correlation_id construction near context.toolCallId to
truncate the value to the existing 256-character remote correlation-ID limit
before returning it, while preserving omission when no toolCallId is present.
Add a focused test covering a 257-character toolCallId and verify the emitted
correlation_id is bounded to 256 characters.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Repository UI

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Run ID: a0977f3f-ef0c-4912-95d4-e282e6f08c98

📥 Commits

Reviewing files that changed from the base of the PR and between a583be3 and 249fce9.

📒 Files selected for processing (3)

• src/agent/conversation/legacy-run-read-adapter.test.ts
• src/tool/remote-mcp.test.ts
• src/tool/remote-mcp.ts

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[kojiwakayama]

Rebased onto current main (e383111c7) and fixed the PR-local lint failure by explicitly narrowing the asserted transport error before reading its message. Verification on rebased head: Deno format and check pass for all three touched files; the focused MCP plus durable replay suite passes 56/56 steps; git diff --check passes. The remaining RequestInit lint errors from the prior run are in three unrelated files, so I am verifying them against the exact current-main CI run rather than folding unrelated changes into this PR.

[kojiwakayama]

The rebased lint failure was the generated veryfront/tool API reference, whose source links moved when the MCP implementation grew. I regenerated the reference with deno task docs; only the four expected source-line links changed. The exact deno task lint:ci run now reaches that generated-reference check cleanly aside from the pre-generation stale-file failure, and all preceding lint/type baselines pass. A fresh exact-head CI run is underway.