Best possible solution:

Update the remaining Matrix docs, then land the shared fallback and regression coverage once exact-head checks are green while leaving broader stale announce freshness on #78055.

Do we have a high-confidence way to reproduce the issue?

Yes. The current-main source path is clear: omitted context plus thread: true reaches resolveThreadBindingSpawnPolicy, whose fallback is currently "fork", and the existing test asserts that fork behavior.

Is this the best way to solve the issue?

Yes, with one docs cleanup. Changing the shared thread-binding policy fallback is the narrowest runtime fix because explicit context: "fork" and configured defaultSpawnContext: "fork" still override it.

Full review comments:

• [P3] Update Matrix thread-binding docs for isolated default — src/channels/thread-bindings-policy.ts:195
  Changing the shared fallback here affects Matrix too: Matrix's subagent hook calls resolveThreadBindingSpawnPolicy, but docs/channels/matrix.md still tells operators to set defaultSpawnContext: "isolated" when they do not want forks. After this merge that instruction is stale; please update the Matrix docs to say isolated is the default and fork is the opt-in.
  Confidence: 0.86

Overall correctness: patch is correct
Overall confidence: 0.88

Acceptance criteria:

• pnpm exec oxfmt --check --threads=1 docs/channels/matrix.md
• pnpm check:docs
• git diff --check

What I checked:

• Current main forks omitted thread-bound context: On current main, resolveThreadBindingSpawnPolicy falls back to "fork" when no account/channel/session defaultSpawnContext is configured, and resolveSubagentContextMode uses that policy for thread-requested native subagent spawns. (src/channels/thread-bindings-policy.ts:191, 95a1c915312a)
• PR changes the shared fallback: The PR diff changes the final fallback from "fork" to "isolated" and updates the thread-bound spawn regression expectations so omitted thread-bound spawns no longer call forkSessionFromParent. (src/channels/thread-bindings-policy.ts:195, 90619451037f)
• Explicit fork remains available: The precedence order still honors account, channel, and session defaultSpawnContext before the fallback, and the existing explicit context: "fork" test remains in the touched context-mode test file. (src/agents/subagent-spawn.context.test.ts:52, 90619451037f)
• Related live proof exists on superseded source PR: The superseded source PR body includes copied terminal output from a live OpenClaw 2026.5.6 config showing after-fix Discord and Telegram thread-bound spawn policies resolve defaultSpawnContext to "isolated"; the earlier ClawSweeper review accepted that terminal proof.
• Remaining Matrix docs drift: Matrix uses resolveThreadBindingSpawnPolicy for native thread-bound subagent spawns, but docs/channels/matrix.md still tells operators to set defaultSpawnContext: "isolated" when they do not want forks, which becomes stale after the shared fallback changes. (docs/channels/matrix.md:562, 95a1c915312a)

Likely related people:

• @steipete: Recent GitHub path history shows several central changes to thread-bound session spawning and subagent spawn hardening in src/channels/thread-bindings-policy.ts, src/agents/subagent-spawn.ts, and docs/tools/subagents.md. (role: recent maintainer and introducer of adjacent behavior; confidence: high; commits: 8612af754b4d, 4f31cbbf553c, 10b89a3b5552; files: src/channels/thread-bindings-policy.ts, src/agents/subagent-spawn.ts, docs/tools/subagents.md)
• @vincentkoc: Recent history includes subagent announcement documentation/output work, and the local checkout blame points to this maintainer across the currently materialized affected files. (role: adjacent maintainer; confidence: medium; commits: e80de466e5e1, a6159bb60d0f; files: docs/tools/subagents.md, src/channels/thread-bindings-policy.ts, src/agents/subagent-spawn.ts)
• @shakkernerd: Recent path history on src/channels/thread-bindings-policy.ts includes thread placement metadata maintenance adjacent to the policy helper changed here. (role: adjacent channel/thread placement owner; confidence: medium; commits: 8b32c3125296; files: src/channels/thread-bindings-policy.ts)

Remaining risk / open question:

• Matrix thread-binding docs remain stale until the low-priority review finding is addressed.
• The PR body reports pnpm tsgo:test:src failing on unrelated current-main OpenAI fixture typing, so exact-head typecheck proof still needs maintainer/CI validation before merge.

Codex review notes: model gpt-5.5, reasoning high; reviewed against 95a1c915312a.