Existing instances stuck on a non-tool-capable model — #100 fix only covers NEW agents, and there's no way to change an instance's model

[serge-ivo]

The live residual behind the (correctly) closed #100. 2641bb0 fixed the root cause for newly created agents — but two gaps remain and together they strand every pre-fix instance:

1. Pre-fix instances are frozen on 3B. An instance copies the model into its DO state at subscribe and never re-reads the template. So an instance created before 2641bb0 (e.g. the live lead-finder 933ebec5) is still on @cf/meta/llama-3.2-3b-instruct → useTools=false → its chat cannot call query_records and confabulates from prose. Reproduced 2026-08-02: asked for exact counts, it returned 106 / 89 (truth: 116 / 95) and said "I did not query a structured collection."
2. No programmatic path to change an instance's model. get_instance_state is read-only; set_instance_settings only touches declared settings; update_agent changes the template, not existing instances (verified — the instance stayed 3B after the template flip). Only the console PUT /v1/instances/:id/state can do it. So the Non-tool-capable default model silently disables ALL tools — a structured-data agent can't query its own collection #100 fix can't be automated or applied to existing instances.

Ask

• A path to set/upgrade an existing instance's model (MCP set_instance_model / a state-write tool, or a one-shot "re-sync from template" action).
• Optionally: a migration/backfill to move pre-fix instances declaring tools off the non-tool-capable default.

Blocks the lead-finder chat today. Relates to closed #100, agent-repo #2.

[serge-ivo]

Shipped the primary ask in d3b892c.

New MCP tool set_instance_model — the programmatic path to change an existing instance's model (was only possible via the console PUT /v1/instances/:id/state). Write-scoped + audited; PUTs {model} to the instance /state route, which handleUpdateState already persists (state.model = updates.model). Its description lists the tool-capable CF models (@cf/meta/llama-4-scout-17b-16e-instruct etc.) and notes BYOK Anthropic. +3 tests (writes to /state, rejects empty, write-scope gated); MCP suite green (41).

So an operator can now move a stuck instance (e.g. the lead-finder 933ebec5) onto a tool-capable model in one MCP call.

On the functional symptom: the chat loop already upgrades a tool-declaring instance off a non-tool-capable model per turn at runtime (agent-think.ts resolveModelForTools, wired from #100), so a pre-fix instance's chat should call its tools rather than confabulate. set_instance_model is the durable operator escape hatch on top of that.

The optional bulk backfill (ask #2) stays deferred — the per-turn upgrade covers the live behaviour and the tool covers the manual case; a migration touching every instance DO is heavier and can be filed separately if we want persisted (not per-turn) upgrades.