fix(mcp): harden OAuth request isolation

[waleedlatif1]

Summary

• keep configured MCP headers off OAuth discovery and token requests
• cancel queued OAuth refresh-lock waits without abandoning an active token rotation
• preserve ambiguous Redis lock ownership while a fail-open refresh runs

Type of Change

• Bug fix

Testing

• 298 tests across 15 MCP, OAuth, execution, and cancellation suites
• bun run lint
• bun run check:audits
• bun run docs-manifest:check
• bunx tsc --noEmit --incremental false

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated
docs	Skipped		Sep 4, 2026 7:51pm UTC

[greptile-apps]

Greptile Summary

This PR hardens MCP OAuth request isolation and cancellation behavior.

• Applies configured MCP headers only to the exact MCP endpoint, keeping them off OAuth discovery and token requests.
• Makes queued and cross-process refresh-lock waits abort-aware without interrupting an already-running token rotation.
• Preserves uncertain lock ownership when an acquire fails open, resolving the previously reported refresh-serialization race.
• Adds focused coverage for header scoping, queued cancellation, and ambiguous lock acquisition.

Confidence Score: 5/5

The PR appears safe to merge; no actionable new failure remains, and the previously reported refresh-serialization issue is fully fixed.

The current implementation preserves a potentially acquired owner token while an uncoordinated refresh runs, releases it only when cancellation prevents the refresh callback, and scopes configured MCP headers away from OAuth endpoints. Abort checks prevent cancelled queued work from executing without abandoning an already-running token rotation.

Important Files Changed

Filename	Overview
apps/sim/lib/mcp/client.ts	Routes OAuth transports through endpoint-scoped fetch handling while retaining configured headers for non-OAuth transports.
apps/sim/lib/mcp/oauth/coordinated-fetch.ts	Restricts configured headers to the MCP endpoint and propagates request cancellation into refresh-lock coordination.
apps/sim/lib/mcp/oauth/storage.ts	Adds abort-aware queue and Redis-lock waiting while preserving ambiguous lock ownership for fail-open refreshes.
apps/sim/lib/mcp/client.test.ts	Verifies OAuth transports use scoped fetch injection rather than globally applied request initialization.
apps/sim/lib/mcp/oauth/coordinated-fetch.test.ts	Covers endpoint-only header application and prompt cancellation while waiting for refresh serialization.
apps/sim/lib/mcp/oauth/storage.test.ts	Covers queued cancellation and both fail-open and cancelled ambiguous-acquire ownership behavior.

Sequence Diagram

sequenceDiagram
    participant Client as MCP client
    participant Scope as Endpoint-scoped fetch
    participant MCP as MCP endpoint
    participant Lock as OAuth refresh lock
    participant OAuth as OAuth discovery/token endpoint

    Client->>Scope: MCP request with configured headers
    Scope->>MCP: Forward request with configured headers
    MCP-->>Client: 401 OAuth challenge
    Client->>Lock: Wait for serialized refresh
    alt Request is cancelled while queued
        Lock-->>Client: Reject without running refresh
    else Lock acquired or safely falls open
        Lock->>OAuth: Discovery/token request without configured MCP headers
        OAuth-->>Lock: Updated credentials
        Client->>Scope: Retry MCP request
        Scope->>MCP: Forward with configured headers
    end

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[cubic-dev-ai]

All reported issues were addressed across 6 files

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[waleedlatif1]

@greptile

[waleedlatif1]

@cubic-dev-ai review this PR

[cubic-dev-ai]

@cubic-dev-ai review this PR

@waleedlatif1 I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

No issues found across 6 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[waleedlatif1]

@greptile

[waleedlatif1]

@cubic-dev-ai review this PR

[cubic-dev-ai]

@cubic-dev-ai review this PR

@waleedlatif1 I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

No issues found across 6 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}