fix(chat-message): refresh stale session-agent cache from explicit input.agent

[sjawhar]

Summary

• When chat.message receives a turn with an explicit input.agent, update the session-agent map unconditionally instead of ignoring it if a value already exists
• Adds regression test covering the stale-agent scenario

Problem

setSessionAgent uses first-write-wins semantics (only writes if key doesn't exist). This is correct for initial session setup (e.g. delegate-task/sync-task.ts initializing a subagent session), but wrong in chat-message.ts where OpenCode delivers an explicit agent on every turn.

Any programmatic caller that sends session.promptAsync with an explicit agent to an already-initialized session gets silently ignored — the session stays pinned to whatever agent was set first. This affects:

• External integrations calling the session API directly
• opencode run with --agent on a resumed session
• Any MCP client interacting with existing sessions

Normal TUI agent switching is unaffected (it goes through different code paths).

Changes

src/plugin/chat-message.ts (2 lines)

• Import updateSessionAgent alongside existing setSessionAgent
• Replace setSessionAgent(input.sessionID, input.agent) with updateSessionAgent(input.sessionID, input.agent)

src/plugin/chat-message.test.ts (13 lines)

• New test: pre-sets agent to "hephaestus", sends turn with input.agent = "build", verifies map updates to "build"

---

Summary by cubic

Refreshes the session agent on every turn when input.agent is provided, replacing first-write-wins behavior in the chat-message handler. Prevents sessions from sticking to the first-set agent for external callers, resumed sessions with --agent, and MCP clients.

• Bug Fixes
  • Replace setSessionAgent with updateSessionAgent in src/plugin/chat-message.ts.
  • Add regression test to ensure agent updates from "hephaestus" to "build".

^{Written for commit 7351a53. Summary will update on new commits.}

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

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

_{Auto-approved: Fixes a stale cache bug by allowing explicit agent updates in the chat handler. The change is localized, logically sound, and includes a regression test.}