fix(opencode): defer defaultAgent lookup in session summarize

[1837620622]

Issue for this PR

Closes #29277

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

/compact (and POST /session/:id/summarize) silently fails when default_agent is configured as a subagent.

Root cause (still on dev):

const defaultAgent = yield* agentSvc.defaultAgent()  // throws if subagent
const currentAgent = messages.findLast(...)?.info.agent ?? defaultAgent

Agent.defaultAgent() throws "default agent X is a subagent" before the last user-message agent fallback is considered. The TUI often ignores the API error, so compaction appears to do nothing.

Minimal fix: only resolve defaultAgent() when the session has no user-message agent:

const lastUserAgent = messages.findLast(...)?.info.agent
const currentAgent = lastUserAgent ?? (yield* agentSvc.defaultAgent())

When the session already has a valid agent (normal case after chatting), /compact works even if default_agent is a subagent. Empty sessions still surface the config error.

Source evidence

• packages/opencode/src/server/routes/instance/httpapi/handlers/session.ts — eager defaultAgent() in summarize
• packages/opencode/src/agent/agent.ts — defaultInfo throws when mode === "subagent"

How did you verify your code works?

• Diff-reviewed against /compact command silently fails when default_agent is a subagent #29277 repro path
• Single-site change; success path when last user agent exists no longer calls defaultAgent()
• Empty-session path still falls through to defaultAgent() unchanged

Author

• GitHub: @1837620622 (传康Kk)
• Commit email: 35034498+1837620622@users.noreply.github.com (GitHub-verified noreply)

Checklist

• Linked issue (Closes #29277)
• No unrelated changes

[1837620622]

Review ping 🙏

/compact no longer throws when default_agent is a subagent if the session already has a user agent (#29277). Checks green.

— @1837620622