Ask: Fall back to the parent model when a sub-agent's model fails at call-time

Start with custom_model.rs::resolve_custom_agent_model and agent_tools.rs::validate_and_resolve_subagent_model, then trace the call-time error path through sessionExecutor.ts and session.ts. Reproduce the unavailable-model 400 described in the issue and inspect the ModelCallFailure data in session-events.d.ts. Done means a sub-agent retries once with the parent model after an unavailable-model failure and the turn continues.

Component: @github/copilot-sdk + copilot-agent-runtime — sub-agent model resolution
Status: Behavior does not match the documented CustomAgentConfig.model contract

Model IDs below are redacted placeholders: <unavailable-model> = a model in the catalog but not onboarded / no-capacity for the test user; <parent-model> = the (available) parent session model.

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1. TL;DR

Ask. When a sub-agent's configured model fails at call-time — i.e. when the runtime issues the actual model request to CAPI (the Copilot model API) — with a model-unavailable 400 (unknown / not onboarded / no capacity), the runtime should fall back to the parent session model and retry that sub-agent's call — instead of letting the sub-agent's model call fail.

Why this is the contract, not a feature request. The SDK already documents this fallback: CustomAgentConfig.model "falling back to the parent session model if unavailable" (types.d.ts:1160-1165). We observe it does not happen at call-time.

One-line evidence. A sub-agent set to <unavailable-model> under a <parent-model> parent → the model is sent to CAPI unchanged → CAPI returns 400 Unknown model "<unavailable-model>" → no retry with the parent model → the sub-agent's call fails.

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2. The documented contract

A host application embeds @github/copilot-sdk and dispatches sub-agents via SessionConfig.customAgents: CustomAgentConfig[]. Each sub-agent's model is a string on CustomAgentConfig.model, documented as:

// @github/copilot-sdk/dist/types.d.ts:1160-1165
/**
 * Model identifier for this agent (e.g. "claude-haiku-4.5").
 * When set, the runtime will attempt to use this model for the agent,
 * falling back to the parent session model if unavailable.
 */
model?: string;

Three cases follow; we rely on the third:

model value	Documented behavior
set to a valid model	used for the sub-agent
omitted / undefined	sub-agent inherits the parent session model
set but unavailable	fall back to the parent session model and continue

The third row is what fails at call-time.

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3. The gap (observed)

For a sub-agent model that is unavailable through the user's CAPI endpoint — e.g. not onboarded or no capacity (<unavailable-model> in our repro) — we observe in the outgoing request and in the event stream:

1. The configured model is sent to CAPI unchanged (logged: the sub-agent's first request carries <unavailable-model>).
2. CAPI returns 400: Unknown model "<unavailable-model>". Please ensure the model is onboarded.
3. The runtime emits an ephemeral, telemetry-only model.call_failure (source=subagent, statusCode=400, model="<unavailable-model>").
4. No fallback: no call is re-issued with the parent model. The sub-agent's model call fails.

Confirmed gap: the documented "fall back to the parent session model if unavailable" is not honored for a call-time 400. (Our source-level reading of why is in Appendix §6.3 — hypothesis only.)

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4. Evidence

4.1 Setup

• A host embedding @github/copilot-sdk; a local end-to-end run.
• Parent session model: <parent-model>.
• Three sub-agents dispatched via SessionConfig.customAgents:
  • A, C → <unavailable-model> (unavailable → 400)
  • B → <parent-model> (available → success)
• All sub-agent event hooks, model.call_failure, and the model sent on each request are instrumented.

4.2 Event sequence

1. subagent.started ×3 (A, B, C).
2. First model request per sub-agent: A→<unavailable-model>, B→<parent-model>, C→<unavailable-model>.
3. model.call_failure ×2 (A, C) — ephemeral / telemetry-only:

   {
     "source": "subagent",
     "initiator": "sub-agent",
     "statusCode": 400,
     "model": "<unavailable-model>",
     "errorMessage": "...Unknown model \"<unavailable-model>\". Please ensure the model is onboarded.",
     "agentId": "call_xxx"
   }
   

   (agentId is the Task tool-call ID, not the sub-agent name.)
4. No parent-model retry follows. The sub-agent's call fails.

4.3 Downstream behavior after the 400 — not confirmed

Our probe covered only background/detached dispatch. We did not verify what happens after the 400 on the normal foreground path, so we make no claim about whether the failure stays isolated to the sub-agent or affects the parent session. Appendix §6.3 records a source-level hypothesis only.

4.4 Why the host cannot recover this itself

• model.call_failure is ephemeral telemetry (session-events.d.ts:2821-2887, ephemeral: true); we found no in-turn hook that lets the host intercept the 400 and retry the sub-agent with another model within the same turn.
• Per the SDK types, customAgents is applied when a session is created or resumed (types.d.ts — SessionConfigBase.customAgents, inherited by ResumeSessionConfig). The host therefore cannot change the model during the failing turn — only after resuming the session, which is too late to recover the first turn. So the fallback must occur inside the runtime.

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5. What we're asking

1. Honor the documented fallback at call-time.
When CAPI rejects a sub-agent's CustomAgentConfig.model as unavailable (400), retry that sub-agent's call once with the parent session model, then continue the turn — matching the model? contract.

The runtime can identify this condition from the failure data already present: statusCode plus the provider error / structured badRequestKind (session-events.d.ts:2821-2887). A one-shot fallback scoped to the sub-agent is sufficient.

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6. Appendix

6.1 Minimal repro

1. Configure a sub-agent with CustomAgentConfig.model = a model that is in the catalog but not onboarded / no-capacity for the test user (<unavailable-model>; parent <parent-model>).
2. Dispatch it.
3. Observe model.call_failure (400) with no parent-model retry; the sub-agent's call fails.

6.2 Key references

• @github/copilot-sdk/dist/types.d.ts:1160-1165 — CustomAgentConfig.model fallback contract.
• @github/copilot-sdk/dist/generated/session-events.d.ts:2821-2887 — ModelCallFailureEvent / ModelCallFailureData (ephemeral: true, initiator="sub-agent", statusCode, model, errorMessage).
• Observed call-time 400: Unknown model "<unavailable-model>". Please ensure the model is onboarded.

6.3 Our runtime-source reading (hypothesis — please verify)

Disclaimer: this is our interpretation of copilot-agent-runtime, offered only as a starting pointer. We may be reading the wrong path. Please verify. The ask in §5 stands on the observed behavior (§3 / §4) regardless.

On why no fallback fires (§3). Sub-agent model selection appears to run through one of two functions, and both seem to fail open (no fallback) when the host supplies no populated available_models list:

• custom_model.rs::resolve_custom_agent_model — empty available_models → first candidate returned verbatim ("CAPI will validate"), no check, no fallback (:236-260); non-empty + unresolved → falls back to session model (:317-328, the only fallback path we see).
• agent_tools.rs::validate_and_resolve_subagent_model — empty → model verbatim (:171-173); non-empty + unresolvable → Error, no fallback (:182-190).
• available_models is populated from the CAPI session bootstrap (model/session.rs:19,96; model/capi_client.rs:1587-1628); an embedded host appears not to feed a catalog, which would leave the list empty on our client.
• Net (our reading): with an empty list the configured model goes straight to CAPI with no check and no fallback → 400. A populated list may still not fix it — membership is not a capacity check, so an in-list but no-capacity model would still 400 at call-time. This is why we ask for a call-time fallback (§5).

On the foreground path (§4.3). The source suggests the normal foreground path degrades gracefully: SessionAgentExecutor.run() catches a sub-agent turn error and emits subagent.failed, returning the failure to the parent as a tool result so the turn continues (sessionExecutor.ts:282-286; session.ts:12364-12382). We have not verified this end-to-end.