fix(server): Claude threads no longer inherit the launching Claude session or race MCP connects by juliusmarminge · Pull Request #25976 · pingdotgg/t3code

juliusmarminge · 2026-10-27T14:09:02Z

Problem

Claude threads fail to share a prompt cache across new threads, for two reasons.

When T3 is started from inside a Claude Code session, every claude it spawns inherits that session's environment: CLAUDECODE, CLAUDE_CODE_ENTRYPOINT=claude-desktop, the session id, the messaging socket and token, CLAUDE_EFFORT, CLAUDE_PID, and so on. The spawned process runs as a child of a session it does not belong to.
Claude Code connects MCP servers without blocking by default. A server that is still connecting when a turn's first request goes out is left out of that request's tool list and added on a later one. Each change rewrites the whole ~52k-token cached prefix, and new threads whose first tool list differs each write a fresh one.

Reproduced on main e9236274a3 with the dev server launched under a fake host session's variables, a Claude instance whose binaryPath logs its env, and a user-scope stdio MCP server that connects after 0.5 to 4 s. Every probe and turn received CLAUDE_CODE_ENTRYPOINT=claude-desktop and the host session's variables. Of four new threads, one went out with the slow server's tools and wrote 52,691 tokens of fresh cache. In the others the tools only arrived on the second request, and that request rewrote the prefix.

Change

mergeClaudeProcessEnvironment in ClaudeHome.ts removes a fixed list of Claude Code host-session markers from the host environment, then merges the instance's own environment as before. A value set on the instance still wins.
The adapter (turns) and the driver (version and capability probes, text generation) build their env with it. The driver now reads HostProcess.Environment like the adapter does, instead of process.env.
Turns default MCP_CONNECTION_NONBLOCKING to false unless the host or instance env sets it. The first request then waits for MCP servers to connect, so every new thread sends the same tool list.
User configuration such as CLAUDE_CONFIG_DIR, ANTHROPIC_*, proxy and telemetry variables still passes through.

Scope and approval

Fixes #25959

Verification

ClaudeAdapterV2.test.ts, "ClaudeAdapterV2 turn environment":
- does not leak a parent Claude Code session into the spawned claude fails on main with CLAUDECODE: "1", CLAUDE_CODE_ENTRYPOINT: "claude-desktop", the session id, socket and effort all present. It passes with the fix, and it fails again if CLAUDE_CODE_ENTRYPOINT is removed from the scrub list.
- connects MCP servers before the first request unless the instance opts out fails on main with [undefined, "true"]. It passes with the fix.
ClaudeAdapterV2.test.ts, ClaudeHome.test.ts and ClaudeTextGeneration.test.ts: 258 passed. vp run check:changed passes.
Runtime transcript: real dev server and real Claude Code 2.1.303 (Sonnet 5.5), the same prompt in each new thread, a fresh database on each side.

	Before, main `e923627`	After, `f072531`
Spawned turn env	`CLAUDECODE=1`, `CLAUDE_CODE_ENTRYPOINT=claude-desktop`, host `CLAUDE_CODE_SESSION_ID`, `CLAUDE_CODE_MESSAGING_SOCKET`/`TOKEN`, `CLAUDE_EFFORT`, `CLAUDE_PID`, `CLAUDE_CODE_CHILD_SESSION`	`CLAUDE_CODE_ENTRYPOINT=sdk-ts` (set by the SDK), `MCP_CONNECTION_NONBLOCKING=false`, no host-session variables in any of the 7 spawns
First-request tool list, jittered slow MCP server	varies per thread: 91 tools without the slow server, or 98 with it	the same 98 tools in all 5 threads
Cache writes on new threads	B1 52,691 fresh; B2 3,652 rewritten on request 2; thread A1 rewrote 53,232 on request 2 when the server connected	D1 52,274 once; D2 to D5 read it (0 written on request 1)

Transcripts: before-main-e9236274a3.txt, after-fix-f072531c75.txt. Scripts: claude-wrap (env-logging binaryPath), slow-mcp.mjs (stdio MCP server with a jittered initialize), shapes.py (reads request shapes and cache usage from the transcripts).

Not checked: the macOS open -a launch from a Claude Desktop session, and claude.ai connectors. The run used a user-scope stdio server through the hub API on Linux; the code path does not depend on the platform.

Model and harness: Claude Opus 5.5 in Claude Code, run as a T3 Code issue lead, with Claude Opus 5.5 and GPT-6.1 Sol step agents.

🤖 Generated with Claude Code

…Code session env These tests reproduce #25959. The env createClaudeAdapterV2 passes to the SDK query still carries a parent Claude Code session's variables (CLAUDECODE, CLAUDE_CODE_ENTRYPOINT, CLAUDE_CODE_SESSION_ID, CLAUDE_CODE_MESSAGING_SOCKET, CLAUDE_EFFORT), and never sets MCP_CONNECTION_NONBLOCKING=false, so MCP tools can miss a turn's first request. Co-Authored-By: Claude Opus 5.5 <[email protected]>

…hat launched T3 Every Claude spawn path (turns, capability and version probes, text generation) strips the host Claude Code session markers before the instance environment is merged, so a value set on the instance still wins. Turns default MCP_CONNECTION_NONBLOCKING to false so new threads send a stable tool list on their first request and share one prompt cache. Fixes #25959 Co-Authored-By: Claude Opus 5.5 <[email protected]>

…only overridden Co-Authored-By: Claude Opus 5.5 <[email protected]>

macroscopeapp · 2026-10-27T14:11:48Z

Approvability

Verdict: Needs Human Approval

Macroscope's review found this PR requires human approval — Claude threads now default to blocking MCP connections, while retaining environment overrides. Human review is needed to accept the default tradeoff between stable tool availability and potentially longer connection waits.

^{You can add or adjust custom eligibility rules. Learn more.}

github-actions · 2026-10-27T14:14:35Z

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	21.5 KiB	21.5 KiB	0 B (0.0%)	24.4 KiB	✅
Codex	Live turn messages	1	1	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.9 KiB	21.9 KiB	0 B (0.0%)	24.4 KiB	✅
Claude	Live turn messages	1	1	0 (0.0%)	8	✅

Baseline: b883156 · PR result: 747e7ca · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

Codex decoded thread snapshot: 108.5 KiB
Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

coderabbitai · 2026-10-27T14:16:03Z

No actionable comments were generated in the recent review.

Walkthrough

The Claude adapter now sets MCP_CONNECTION_NONBLOCKING to "false" by default. Provider-instance environment values override this default, and tests verify both cases.

Changes: Claude MCP connection environment

Layer / File(s)	Summary
Set and test MCP connection default `apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts`, `apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts`	The adapter sets `MCP_CONNECTION_NONBLOCKING` to `"false"` before applying provider-instance environment values. Tests capture full query environments and check the default and explicit override.

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Possibly related issues

Issue 31200 concerns Claude failures during MCP registration. This change sets the default MCP connection behavior, but does not change error reporting or session state.

Suggested reviewers: maria-rcks, saphid

Merge Risk: Minimal · up to 747e7

Claude turns now default to waiting for MCP servers to connect before the first request. Instance settings can still opt out. No merge-blocking risk was found.

Pre-merge checks: 4

Check name	Status	Explanation
Title check		The title clearly and concisely describes both primary changes: Claude session environment isolation and blocking MCP connections.
Description check		The description includes all required sections. It explains the problem, implementation, scope, focused tests, observed results, limitations, and agent details.
Linked Issues check		Direct issue `#25959` is closed and supplies historical context only. No active directly linked issue provides coding requirements for this pull request. The reported environment and MCP changes therefo…
Out of Scope Changes check		The reviewed changes set the Claude turn environment and add focused environment-capture and MCP override coverage. These changes address the historical `#25959` problem. The available change summary sh…

Full details: Linked Issues check

Explanation

Direct issue #25959 is closed and supplies historical context only. No active directly linked issue provides coding requirements for this pull request. The reported environment and MCP changes therefore do not fail a current linked-issue requirement.

Full details: Out of Scope Changes check

Explanation

The reviewed changes set the Claude turn environment and add focused environment-capture and MCP override coverage. These changes address the historical #25959 problem. The available change summary shows no unrelated product behavior or unrelated file changes.

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Repository: pingdotgg/t3code

Branch: fix/issue-25959

File: .coderabbit.config.ts

Commit: 747e7ca

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Run ID: `25c99f31-75ed-4e97-a833-0271a0bfd2ef`

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Reviewing files that changed from the base of the PR and between `134bf64` and `747e7ca`.

Files selected for processing: 2

`apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts`

`apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts`

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juliusmarminge · 2026-11-09T21:42:13Z

Thanks for the thorough investigation and fix. #29169 (47f2c8d29e) covers the inherited host-session environment. Could you cut this PR down to the separate blocking MCP connection behavior and its tests? The startup latency tradeoff noted in the merged PR still needs consideration.

# Conflicts: # apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts # apps/server/src/provider/Drivers/ClaudeDriver.ts

juliusmarminge · 2026-11-11T01:20:30Z

The automated pipeline stopped on this pull request and will not merge it:

someone else pushed to fix/issue-25959 (head is now 134bf64); the pipeline does not build on commits it did not review

It stays open for a person to finish or close.

The merge with main kept main's environment sanitizing and dropped this branch's blocking MCP connect default. Restore it on top of main's environment, keeping instance and host overrides, and cover it with a test. AI-assisted; verified with the tests and tools listed above.

juliusmarminge and others added 3 commits October 27, 2026 13:38

test(server): assert the inherited Claude entrypoint is dropped, not …

134bf64

…only overridden Co-Authored-By: Claude Opus 5.5 <[email protected]>

github-actions Bot added vouch:trusted Author is trusted by repo permissions or the VOUCHED list. size:M 30-99 changed lines (additions + deletions). labels Oct 27, 2026

juliusmarminge added bugfix Fix PR opened by an issue lead; enters the PR loop pr:gate-failed A land gate failed labels Oct 28, 2026

theogmiguel mentioned this pull request Nov 1, 2026

fix(server): Claude threads no longer inherit the launching Claude session or race MCP connects theogmiguel/houston#515

Merged

t3dotgg mentioned this pull request Nov 2, 2026

fix(server): Claude threads no longer join the Claude session that launched T3 #29169

Merged

Merge remote-tracking branch 'origin/main' into fix/issue-25959

a5a089c

# Conflicts: # apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts # apps/server/src/provider/Drivers/ClaudeDriver.ts

juliusmarminge added 2 commits November 11, 2026 02:19

Merge remote-tracking branch 'origin/main' into fix/issue-25959

4501c04

github-actions Bot added size:XS 0-9 changed lines (additions + deletions). and removed size:M 30-99 changed lines (additions + deletions). labels Nov 11, 2026

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fix(server): Claude threads no longer inherit the launching Claude session or race MCP connects - #25976

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