fix(runtime-core): avoid MCP tool discovery after agent cancellation (#450) by tkislan · Pull Request #457 · deepnote/deepnote

tkislan · 2026-08-14T06:40:38Z

Fixes #450

Problem

When an agent run is cancelled while MCP clients are still being created or while client.tools() is in flight, the handler continued into tool discovery and/or model streaming after the abort signal had already fired.

Changes

`agent-handler.ts`

Add context.signal?.throwIfAborted() after retaining fulfilled MCP clients from Promise.allSettled, before checking for creation failures (abort wins over spawn errors).
Add a second guard after client.tools() completes, before constructing the agent and starting the model stream.
Existing finally cleanup is unchanged.

`execution-engine.ts`

Add optional ExecutionOptions.signal forwarded only to AgentBlockContext.signal.
Does not interrupt code/SQL/kernel blocks; runProject / runFile still resolve with a failed agent block when the signal aborts.

Tests

Race tests for abort during pending createMCPClient and pending client.tools().
Engine forwarding tests and behavioral test that abort resolves with failedBlocks === 1.

Docs

Document signal in packages/runtime-core/README.md.

Verification

pnpm exec vitest run packages/runtime-core/src/execute-agent-block.test.ts packages/runtime-core/src/execution-engine.test.ts
pnpm test
pnpm typecheck
pnpm biome:check

Summary by CodeRabbit

New Features
- Added optional cancellation support for agent block execution.
- Aborted agent runs now stop setup promptly and report the cancellation reason.
- Aborted agent blocks are handled as failed blocks without causing the overall project run to reject.
Documentation
- Documented signal forwarding, scope, and cancellation behavior.

Add abort checks after MCP client creation and tool discovery so cancelled agent runs skip merging tool sets. Expose optional signal on ExecutionOptions and document the agent-only contract in README.

…450 Add MCP cancellation race tests in execute-agent-block.test.ts and ExecutionEngine signal pass-through / failed-block summary tests.

coderabbitai · 2026-08-14T06:41:05Z

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: f09fa8f8-2a43-4adc-8326-0416a0d0b247

📥 Commits

Reviewing files that changed from the base of the PR and between d20bc24 and b1ec9db.

📒 Files selected for processing (5)

packages/runtime-core/README.md
packages/runtime-core/src/agent-handler.ts
packages/runtime-core/src/execute-agent-block.test.ts
packages/runtime-core/src/execution-engine.test.ts
packages/runtime-core/src/execution-engine.ts

📝 Walkthrough

Walkthrough

The execution engine now accepts an optional AbortSignal and forwards it to agent blocks. Agent setup checks for cancellation after MCP client creation and tool discovery. Aborted agent runs preserve client cleanup, skip tool or model execution, and are recorded as failed blocks without rejecting runProject. Documentation and tests cover signal propagation, cancellation timing, cleanup, and outcomes.

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

Merge Risk: ⚪ Minimal · up to b1ec9

The PR changes runtime cancellation behavior and adds related tests and documentation; no actionable merge-blocking risk remains based on the supplied evidence.

Possibly related PRs

deepnote/deepnote#342: Introduced the agent execution flow modified here.
deepnote/deepnote#445: Added related agent cancellation handling.
deepnote/deepnote#452: Modified MCP client lifecycle handling in executeAgentBlock.

Suggested reviewers: m1so, dinohamzic

🚥 Pre-merge checks | ✅ 6

✅ Passed checks (6 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the primary fix: preventing MCP tool discovery after agent cancellation.
Linked Issues check	✅ Passed	The changes satisfy the cancellation, cleanup, signal propagation, and regression-test requirements in [`#450`].
Out of Scope Changes check	✅ Passed	The README, signal forwarding, implementation, and tests directly support the cancellation behavior in [`#450`].
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Updates Docs	✅ Passed	The PR adds runtime-core README documentation for `signal`, scope, forwarding, and abort results. The private roadmap repo is not visible here; update its landing page separately.

_{Comment @coderabbitai help to get the list of available commands.}

codecov · 2026-08-14T06:42:29Z

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.30%. Comparing base (3a3a15c) to head (b1ec9db).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #457   +/-   ##
=======================================
  Coverage   88.30%   88.30%           
=======================================
  Files         191      191           
  Lines       10697    10699    +2     
  Branches     2996     2996           
=======================================
+ Hits         9446     9448    +2     
  Misses       1249     1249           
  Partials        2        2

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tkislan added 2 commits August 14, 2026 06:37

feat(runtime-core): forward abort signal to agent block execution

3175207

Add abort checks after MCP client creation and tool discovery so cancelled agent runs skip merging tool sets. Expose optional signal on ExecutionOptions and document the agent-only contract in README.

test(runtime-core): add abort guards and signal forwarding tests for #…

b1ec9db

…450 Add MCP cancellation race tests in execute-agent-block.test.ts and ExecutionEngine signal pass-through / failed-block summary tests.

coderabbitai Bot approved these changes Aug 14, 2026

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fix(runtime-core): avoid MCP tool discovery after agent cancellation (#450) - #457

fix(runtime-core): avoid MCP tool discovery after agent cancellation (#450)#457
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tkislan commented Aug 14, 2026 • edited by coderabbitai Bot Loading Uh oh! There was an error while loading. Please reload this page.

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Problem

Changes

agent-handler.ts

execution-engine.ts

Tests

Docs

Verification

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tkislan commented Aug 14, 2026 •

edited by coderabbitai Bot

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`agent-handler.ts`

`execution-engine.ts`

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