feat(opencode): declare toolCallSpans on /v1/capabilities so agent-runtime can bound a silent model call

src/backends/opencode.ts

@@ -57,6 +57,9 @@ export interface OpencodeBackendOptions {
 export class OpencodeBackend implements Backend {
   readonly name = 'opencode'
   readonly defaultExecutionTimeoutMs: number
+  // The signal plugin reports a call at its start; it is installed in the turn's private config
+  // directory, which every turn with an AgentProfile has (agent-runtime always sends one).
+  readonly toolCallSpans = true
   private readonly spawner: Spawner
   constructor(private readonly opts: OpencodeBackendOptions) {
     this.defaultExecutionTimeoutMs = opts.timeoutMs

src/backends/types.ts

@@ -603,6 +603,12 @@ export interface Backend {
   readonly name: string
   /** Operator-selected fallback used only when `execution.timeoutMs` is absent. Zero/absent means no deadline. */
   readonly defaultExecutionTimeoutMs?: number
+  /**
+   * The stream reports each tool call when it starts and its outcome when it ends, so a caller can
+   * tell a silent model from a running tool. Served as `toolCallSpans` on `/v1/capabilities`;
+   * agent-runtime runs `modelSilenceMs` only on a route that declares it.
+   */
+  readonly toolCallSpans?: boolean
 
   /** Does this backend want to handle requests for `model`? */
   matches(model: string): boolean

src/sessions/retained.ts

@@ -146,14 +146,17 @@ export class RetainedSessionService {
     return parseNativeContinuation(value)
   }
 
-  async capabilities(model: string, signal?: AbortSignal): Promise<AgentEnvironmentCapabilities> {
-    const { capabilities } = await readyBackendCapabilities({
+  async capabilities(
+    model: string,
+    signal?: AbortSignal,
+  ): Promise<AgentEnvironmentCapabilities & { toolCallSpans?: true }> {
+    const { backend, capabilities } = await readyBackendCapabilities({
       registry: this.registry,
       model,
       healthProbeTimeoutMs: this.healthProbeTimeoutMs,
       signal,
     })
-    return capabilities
+    return backend.toolCallSpans ? { ...capabilities, toolCallSpans: true } : capabilities
   }
 
   async create(input: RetainedCreateInput, signal?: AbortSignal): Promise<RetainedSessionView> {

tests/retained-sessions.test.ts

@@ -1077,6 +1077,14 @@ describe('retained Agent Interface sessions', () => {
     })
     expect(capabilities).not.toHaveProperty('nativeContinuation')
     expect(capabilities).not.toHaveProperty('interactions')
+    expect(capabilities).not.toHaveProperty('toolCallSpans')
+  })
+
+  it('declares toolCallSpans only for a backend whose stream reports a call at its start and end', async () => {
+    fixture = setup(Object.assign(new OneShotBackend('opencode'), { toolCallSpans: true }))
+    const response = await fixture.app.request('/v1/capabilities?model=opencode%2Ftest')
+    expect(response.status).toBe(200)
+    expect(await json(response)).toMatchObject({ toolCallSpans: true, streaming: { live: true } })
   })
 
   it('refuses retained capability discovery and creation until native health is ready', async () => {