fix: make spawned subagents follow the parent session's model

[SergioGen]

Problem

Subagents spawned via the spawn_agent tool create sessions through session.prompt without passing a model, so they fall back to the model baked into the agent config at plugin startup. Switching models with /models only affects the running session, so subagents keep using the stale model - after a rate limit they error out even though the user switched models.

Fix

• Capture the live model from each chat.message into a per-session map
• Inject it into the subagent session's session.prompt body, so subagents follow the parent session's model
• Propagate the model to child sessions so nested spawns (e.g. executor -> implementer) keep following it
• Clean up the map entry on session.deleted

Changes

• src/session-model.ts (new): in-memory session model map with get/set/delete accessors
• src/index.ts: capture model in chat.message, clean up on session.deleted
• src/tools/spawn-agent.ts: inject and propagate the parent session model
• tests/session-model.test.ts (new): 3 unit tests for the map helpers

Verification

• tsc --noEmit: pass
• eslint: pass
• biome check: pass
• bun test: not run locally (no bun available) - CI should run it

---

Summary by cubic

Spawned subagents now inherit the parent session’s active model. Previously, spawn_agent sessions ignored /models and fell back to the startup-baked model, which could trigger rate-limit errors.

• Capture the active model from each chat.message, store it in-memory per session, and include it in spawn_agent session.prompt; also set it on the child so nested spawns follow.
• Delete the stored model on session.deleted.
• Add src/session-model.ts and unit tests for get/set/delete of the session model map.

^{Written for commit fb01e92. Summary will update on new commits.}

[cubic-dev-ai]

1 issue found and verified against the latest diff

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="src/tools/spawn-agent.ts">

<violation number="1" location="src/tools/spawn-agent.ts:68">
P2: When an agent has a `micode.json` model override, this request-level `model` replaces that explicit per-agent choice for every spawned run. Resolve the task agent's configured override first, and inherit the parent model only when no per-agent model is configured.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

P2: When an agent has a micode.json model override, this request-level model replaces that explicit per-agent choice for every spawned run. Resolve the task agent's configured override first, and inherit the parent model only when no per-agent model is configured.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At src/tools/spawn-agent.ts, line 68:

<comment>When an agent has a `micode.json` model override, this request-level `model` replaces that explicit per-agent choice for every spawned run. Resolve the task agent's configured override first, and inherit the parent model only when no per-agent model is configured.</comment>

<file context>
@@ -58,15 +59,20 @@ async function executeAgentSession(ctx: PluginInput, task: AgentTask): Promise<s
     body: {
       parts: [{ type: "text", text: task.prompt }],
       agent: task.agent,
+      ...(model ? { model } : {}),
     },
     query: { directory: ctx.directory },
</file context>