# Centralised model registry

> **⚠️ Superseded (2026-05-21).** The model registry is now driven by a
> single manifest — **`.opencode/models/models.yaml`** — with
> `tools/sync_models.py` generating every derived surface (the
> `.opencode/models/*.txt` files, the `opencode.json` `agent` block, and
> each agent's `.md` `model:` frontmatter). `sync_models.py` runs
> automatically at controller-pipeline startup.
>
> See **[`.opencode/models/README.md`](../../.opencode/models/README.md)**
> for the current design and operator workflow. Below, the claim that
> `.opencode/models/*.txt` is the "single source of truth", the
> "No `model:` frontmatter" rule, and the "Why not a renderer / single
> registry file" sections are **obsolete** — `models.yaml` +
> `sync_models.py` are exactly that renderer and single file. The
> tier-ladder content (`tiers.yaml` / `sync_tier_models.py`) and the
> Stage 2 fallback-chain ideas remain accurate.

This document explains where each agent's model assignment lives, how to
swap a model for the next dispatch, and what the planned Stage 2
fallback layer will add. Everything described here is in production as
of the 2026-05-10 "Stage 1 model centralisation" change.

## TL;DR — how to swap a model

```bash
# Switch the dispatcher fleet from Qwen to Sonnet for the next cycle.
echo "anthropic/claude-sonnet-4-6" > .opencode/models/default.txt

# Restart OpenCode so it re-reads opencode.json's {file:...}
# interpolation. (See "Why a restart is required" below.)
kill <opencode-pid>
bash scripts/opencode-builder.sh   # or however you started it

# Next dispatched session uses the new model.
python3 tools/dispatch_review.py --once
```

> **Why a restart is required (Stage 1 limitation).** Stage 1 ships
> with a dispatcher-side runtime override at `POST /session`, but
> OpenCode does **not** propagate the session-level model to the
> per-prompt generation path — every `POST /session/{id}/prompt_async`
> re-resolves the agent's model from `opencode.json`'s
> `agent.<name>.model`, which is loaded into memory at server startup.
> So an edit to `.opencode/models/<x>.txt` only takes effect after
> the next OpenCode restart re-reads the `{file:...}` interpolation.
>
> The dispatcher's `POST /session` override still serves two real
> purposes:
>
> 1. **Observability** — every dispatched session record carries the
>    intended model, so the session list shows what the registry
>    *meant* to use.
> 2. **Drift detection** — when the override on the session record
>    disagrees with what OpenCode actually used for generation,
>    operators have evidence the static cache is stale.
>
> True restart-free swaps are reserved for Stage 2 (see "Stage 2"
> section).

## Where each model is defined

| File | Purpose |
|---|---|
| `.opencode/models/*.txt` | **Single source of truth.** One file per role; each contains exactly one `providerID/modelID` line. |
| `opencode.json` → `agent.<name>.model` | References the corresponding `.opencode/models/<name>.txt` via `{file:...}` interpolation. OpenCode reads this at startup to populate its static agent registry. |
| `.opencode/agents/*.md` | **No `model:` frontmatter.** Stripped during Stage 1 — keeping a `model:` line here would create two sources of truth with undocumented precedence. |
| `tools/_opencode_worker.py` → `_resolve_role_model(agent_name)` | Reads `.opencode/models/<agent_name>.txt` first, falls back to `default.txt`, returns a `ResolvedModel` (or `None`). |

The registry file naming convention matches the agent name 1:1, with
`default.txt` as the catch-all for agents that share a worker model
(today: the bulk of `git-*-util`, `session-health-*-util`, the
dispatcher entry-point workers, etc.). Tier selectors (`tier-min`,
`tier-0`, `tier-1`, `tier-2`) each have their own `.txt` file so
swapping one slot's pinned model is independent. **Tier `.txt` files
are generated** from `.opencode/models/tiers.yaml` by
`tools/sync_tier_models.py` — see `.opencode/models/README.md` for
the operator workflow.

## Lookup precedence

The resolver tries two paths, in order:

1. `.opencode/models/<agent-name>.txt` — per-agent override.
2. `.opencode/models/default.txt` — fallback.

If neither exists (or both are malformed), the resolver returns
`None`. The worker then creates the session **without** a `model`
field and OpenCode falls back to its own static resolution (the
`agent.<name>.model` from `opencode.json`, then the agent's `.md`
frontmatter, then OpenCode's global default). This is the same fail-
open semantic OpenCode uses for any missing config — the dispatcher
remains functional even with the registry directory absent.

Two agents have NO `model:` line and intentionally do not appear in
the registry:

- `task-implementor` — inherits from the `tier-*` selector that
  invoked it (the entire purpose of the `tier-*/task-*` split).
- `agent-evolution-pool-supervisor` — inherits the same way.

These agents continue to work without explicit assignments.

## File format

Every `.opencode/models/<name>.txt`:

- Contains **exactly one non-empty line**.
- Starts with the provider id, then `/`, then the model id (e.g.
  `anthropic/claude-haiku-4-5`).
- May have a single trailing newline. Leading and trailing whitespace
  on the model line are stripped by the resolver.
- Does NOT support comments, blank lines, or multi-line content.
  Stage 2 will add a separate `<name>.fallbacks.txt` file for the
  fallback chain (see below); for now, multi-line files are rejected
  as malformed.

The provider id must already be declared in `opencode.json`'s
`provider.<name>` block. If you reference an unknown provider, OpenCode
will 400 the `POST /session` call with an opaque message — the cost of
catching this at OpenCode's edge instead of in the resolver.

## How the dispatcher uses the registry

1. `_dispatch_runtime.dispatch_one` calls
   `_opencode_worker.run_session_blocking(agent="pr-review-worker", …)`.
2. `run_session_blocking` calls `_resolve_role_model("pr-review-worker")`.
3. The resolver reads `.opencode/models/pr-review-worker.txt` (absent),
   then `.opencode/models/default.txt`, parses the model line, and
   returns a `ResolvedModel(provider_id="openai",
   full_id="openai/gpt-5-mini")`.
4. `run_session_blocking` builds the `POST /session` body as
   `{"title": "...", "model": {"providerID": "openai", "id":
   "openai/gpt-5-mini"}}` and dispatches. The session record now
   carries the intended model — useful for operator visibility.
5. The actual model used for generation depends on
   `opencode.json`'s `agent.<name>.model` cached at OpenCode startup,
   NOT the session-level override. Stage 1 therefore requires the two
   to agree (which they do as long as the operator restarts OpenCode
   after editing a model file). The override still serves as a
   consistency sentinel — if it disagrees with the assistant
   message's `modelID`, the operator knows the static cache is stale.

Subagents the worker invokes via the Task tool (e.g.
`implementation-worker` → `tier-dispatcher` → `tier-0` →
`task-implementor`) pick their model from OpenCode's static agent
registry — which is itself driven by the same `.opencode/models/*.txt`
files via `{file:...}` interpolation in `opencode.json`.

We tried adding `model` to the `prompt_async` body to bypass the
startup cache. OpenCode 0.x silently dropped those requests (returned
200 OK but never produced an assistant message). Schema for runtime
prompt-time overrides is undocumented and is the open question for
Stage 2.

## Operator workflows

### Swap one role

For non-tier agents (e.g. the default worker, `ca-test-infra-improver`),
edit the `.txt` file directly:

```bash
# Pin ca-test-infra-improver to a newer Haiku release.
echo "anthropic/claude-haiku-5-1" > .opencode/models/ca-test-infra-improver.txt
```

For tier slots (`tier-min`, `tier-0`, `tier-1`, `tier-2`), edit the
manifest and re-run the generator — the `.txt` files are generated,
not hand-edited:

```bash
# 1. Change the `model:` line for the desired slot in tiers.yaml
$EDITOR .opencode/models/tiers.yaml
# 2. Regenerate the .txt files and the tier-dispatcher mapping table
python3 tools/sync_tier_models.py
# 3. Verify everything stays in sync (CI also runs this)
python3 tools/sync_tier_models.py --check
```

Effect (either path): the dispatcher uses the new model on the next
session that invokes the affected agent (or any subagent that calls
into it). Other roles unchanged.

### Swap the bulk worker

```bash
# Move every default-tier agent to a different model.
echo "openai/gpt-5-codex" > .opencode/models/default.txt
```

Effect: every agent in the registry that didn't have its own per-role
file now uses the new value on the next dispatch. The `tier-*` files
are unaffected — they still have their explicit pins.

### Verify what's currently active

```bash
# Print the model the dispatcher would resolve for a given agent name.
python3 -c "
import importlib.util, pathlib
spec = importlib.util.spec_from_file_location(
    '_opencode_worker',
    pathlib.Path('tools/_opencode_worker.py'),
)
mod = importlib.util.module_from_spec(spec)
spec.loader.exec_module(mod)
r = mod._resolve_role_model('pr-review-worker')
print(f'pr-review-worker -> {r.full_id} (from {r.role_file})' if r else 'no override')
"
```

For the static (OpenCode-side) view, hit OpenCode's `/config`
endpoint:

```bash
curl -s http://127.0.0.1:4096/config | python3 -c "
import json, sys
cfg = json.load(sys.stdin)
print(cfg['agent']['pr-review-worker']['model'])
"
```

If the two values disagree after editing a model file, the OpenCode
server has stale config — restart it to re-read the `{file:...}`
interpolation. The dispatcher path is always live.

### Roll back

```bash
git checkout HEAD -- .opencode/models/default.txt
# next dispatched session uses the committed model
```

Because the registry files are committed and tiny, the rollback story
is the same as for any other config: `git checkout`, no migration.

## Inheriting subagents

Two agents intentionally have NO model assignment anywhere
(no `.opencode/models/<name>.txt`, no `model:` in `.md`, no
`agent.<name>.model` in `opencode.json`):

- `task-implementor`
- `agent-evolution-pool-supervisor`

Per [OpenCode's agent inheritance rule](https://opencode.ai/docs/agents#model),
subagents without an explicit model inherit from the agent that
invoked them. `task-implementor` always runs under a `tier-*` parent,
so it gets the tier's model; `agent-evolution-pool-supervisor` runs
under whichever supervisor or entry-point invoked the pool.

Adding model inheritance to more subagents is a future opportunity —
it would let us delete more `.opencode/models/*.txt` files in favour
of cascading from `default.txt` through the call chain — but Stage 1
deliberately preserves today's bit-for-bit semantics, so no other
subagent is converted to inherit yet.

## Stage 2 (planned) — fallback chain + restart-free swaps

Stage 2 has two related goals:

### 2a. Fallback chain (failover)

The Stage 1 design intentionally accommodates fallback as a purely
additive extension. The plan, NOT shipped yet:

1. Add `.opencode/models/<name>.fallbacks.txt` (optional). Multi-line
   ordered list of fallback model ids — first line is tried after the
   primary fails, second line after that, etc.
2. `_opencode_worker.run_session_blocking` gains a candidate loop:
   try the primary, catch a documented set of retryable errors
   (HTTP 4xx with "endpoint paused", 401, 403, 5xx, connection refused,
   DNS NXDOMAIN, timeout), fall through to the next candidate; only
   exhaust to `transport-error` when every candidate is tried.
3. A `model-fallback-engaged` status comment is posted on the PR so
   operators see the live failover.

The Stage 1 file layout already reserves the namespace: a future
`<name>.fallbacks.txt` is the natural companion to today's
`<name>.txt`, and the resolver's "missing file = no override" semantic
extends cleanly to "missing fallbacks file = no fallback chain."

### 2b. Restart-free model swaps

Stage 1's restart requirement comes from OpenCode caching
`agent.<name>.model` at server startup. The clean fix requires one
of:

- **Option A (preferred):** discover the working schema for a
  runtime model override on `POST /session/{id}/prompt_async`. The
  obvious `{providerID, id}` and `{providerID, modelID}` shapes
  cause OpenCode to silently drop the request — diagnostic output
  needed.
- **Option B:** OpenCode adds a `/config/reload` endpoint we can
  call after editing `.opencode/models/`.
- **Option C:** the dispatcher hot-restarts OpenCode automatically
  when it sees a registry mtime newer than the OpenCode process
  start time. This is the lowest-effort path and lets operators
  treat restarts as an implementation detail. Cost: ~5s downtime
  per swap; OpenCode startup takes a second or two.

Until Stage 2 ships, treat the restart as part of the swap procedure.

## Why not a renderer?

Earlier proposals included a `tools/render-agents.py` build step that
would expand a TOML registry into the `model:` lines of each agent
`.md` file. We rejected that path for two reasons:

1. **Two-step swap.** Operators would have to edit one file, run the
   renderer, then restart OpenCode. With Stage 2 a renderer-based
   solution would also need the failover loop wired in twice (in the
   renderer for "static" baselines AND in the dispatcher for
   "dynamic" failover).
2. **Generated files drift.** The renderer would commit generated
   `.md` files that need to stay in sync with the registry. The
   `{file:...}` interpolation in `opencode.json` accomplishes the
   same thing with no generated artefact and no commit drift.

(Stage 1 still requires a restart, so the "no two-step swap"
advantage of the runtime override is partly aspirational — it pays
off fully once Stage 2 lands.)

## Why not a single registry file (e.g. `models.toml`)?

OpenCode's `{file:...}` interpolation reads one entire file's content
as a single string — there's no sub-path syntax like
`{file:./models.toml#tier_haiku}`. A single registry file therefore
requires either a renderer (rejected; see above) or a runtime
override + custom parser. The per-file layout adopted in Stage 1:

- Maps 1:1 to OpenCode's native `{file:...}` capability with no glue.
- Gives each role a distinct `git blame` history.
- Keeps the runtime override trivially small (~25 LOC).

If the registry grows to ~100 roles, consolidating into a TOML +
custom parser is a clean future refactor with no API surface change.

## Test coverage

The Stage 1 contract is tested by
[`tests/auto_agents/test_opencode_worker_models.py`](../../tests/auto_agents/test_opencode_worker_models.py):

- Resolver: per-agent precedence, default fallback, return-None on
  missing files, whitespace stripping, multi-line rejection,
  malformed-line rejection, slash-split correctness for multi-`/`
  model ids.
- `run_session_blocking`: model threaded into `POST /session` body
  when resolver succeeds; field omitted when resolver returns
  `None`; per-agent override wins over `default.txt`; malformed
  registry degrades gracefully.
- Integration: every agent wired in `opencode.json`'s `agent` block
  resolves to a non-`None` model against the production registry.
  Guards against shipping a config that the resolver cannot honour.