[juliusmarminge]

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

[macroscopeapp]

Approvability

Verdict: Needs Human Approval

Macroscope's review found this PR requires human approval — This focused server fix sanitizes inherited Claude session variables and changes the default MCP connection behavior for Claude turns, affecting existing production request paths. Because it changes a product default, the change warrants human review.

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[github-actions]

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	+6 B (+0.1%)	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	1.3 KiB	+6 B (+0.5%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	21.6 KiB	21.6 KiB	0 B (0.0%)	24.4 KiB	✅
Codex	Live turn messages	2	2	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	−29 B (−0.6%)	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	1.3 KiB	1.2 KiB	−29 B (−2.3%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.9 KiB	21.9 KiB	−41 B (−0.2%)	24.4 KiB	✅
Claude	Live turn messages	2	1	−1 (−50.0%)	8	✅

Baseline: e923627 · PR result: 134bf64 · 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]

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Recent review info

Run configuration

• Source mode: Auto
• Repository: pingdotgg/t3code
• Branch: fix/issue-25959
• File: .coderabbit.config.ts
• Commit: 134bf64
• Layers: Repository TS config
• Review profile: CHILL
• Plan: Team
• Run ID: e72d8f0b-fce5-49f3-838f-e53e9ffed727

Commits

Reviewing files that changed from the base of the PR and between e923627 and 134bf64.

Files selected for processing: 4

• apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts
• apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts
• apps/server/src/provider/Drivers/ClaudeDriver.ts
• apps/server/src/provider/Drivers/ClaudeHome.ts

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Walkthrough

Claude environment construction now removes inherited Claude session variables before merging provider-instance values. The adapter and driver use this filtered environment. The adapter also defaults MCP_CONNECTION_NONBLOCKING to "false". Tests check filtering, retained host values, instance overrides, and the default.

Changes: Claude environment handling

Layer / File(s)	Summary
Filter and merge Claude environment apps/server/src/provider/Drivers/ClaudeHome.ts	Adds a helper that removes selected Claude session variables from a copy of the host environment, then merges provider-instance values.
Use filtered environment in Claude execution apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.ts, apps/server/src/provider/Drivers/ClaudeDriver.ts, apps/server/src/orchestration-v2/Adapters/ClaudeAdapterV2.test.ts	The adapter and driver use the Claude-specific environment helper. The adapter sets MCP_CONNECTION_NONBLOCKING to "false" before merging, so an instance value can override it. Tests check filtering, retained host values, instance overrides, and the default.

Priority: Normal

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

Change: Bug fix · Severity of issue fixed: Medium

Suggested labels: bugfix

Suggested reviewers: maria-rcks, saphid

Merge Risk: Minimal · up to 134bf

The environment isolation change is ready to merge after normal checks; no material issue remains established.

Pre-merge checks: 4

Check name	Status	Explanation
Description check		The description includes all required sections. It explains the problem, change, scope approval, focused verification, observed results, limitations, and agent details.
Title check		The title is concise, specific, and accurately summarizes both primary fixes: preventing inherited Claude sessions and avoiding MCP connection races.
Linked Issues check		The PR addresses the active direct issue #25959. mergeClaudeProcessEnvironment removes inherited Claude Code session markers and then merges the instance environment, so instance values remain effec…
Out of Scope Changes check		The diff stays within issue #25959. It changes Claude environment construction, MCP connection timing, and focused automated tests for those behaviors. No unrelated product behavior or unrelated files…

Full details: Linked Issues check

Explanation

The PR addresses the active direct issue #25959. mergeClaudeProcessEnvironment removes inherited Claude Code session markers and then merges the instance environment, so instance values remain effective. The adapter and driver use this environment for turns, probes, and text generation. Turns set MCP_CONNECTION_NONBLOCKING to false by default while preserving an instance or host override. Added tests cover session-variable removal, retained user environment, instance entrypoint override, and MCP opt-out. The reported validation passes.

Full details: Out of Scope Changes check

Explanation

The diff stays within issue #25959. It changes Claude environment construction, MCP connection timing, and focused automated tests for those behaviors. No unrelated product behavior or unrelated files are changed.

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