What causes "failed to set model: %w" in github/copilot-sdk?

Verify the model ID/provider string against the runtime's model list

failed to set model

Error message

failed to set model: %w

What it means

Session's model-switch path (SetModel) builds switch parameters (model, optional provider, auto-tier defaults) and calls s.RPC.Model.SwitchTo. This error wraps any failure of that RPC — unknown model/provider identifier, invalid auto-tier combination, or transport failure.

Solutions

Verify the model ID/provider string against the runtime's model list
Check that the AutoTier value is valid for the chosen provider
Inspect the wrapped error for the runtime's specific rejection reason
Upgrade SDK/runtime together if the model was added in a newer release

Example fix

// before
err := session.SetModel(ctx, "gpt-5-turbo", "")
// after
if err := session.SetModel(ctx, "gpt-5-turbo", ""); err != nil {
    log.Printf("set model failed: %v — check model/provider IDs", err)
}

Defensive patterns

Strategy: validation

Validate before calling

var validModels = map[string]bool{"claude-sonnet-4": true /* ...from runtime list */}
if !validModels[modelID] {
    return fmt.Errorf("unknown model %q", modelID)
}

Try / catch

if err := session.SetModel(ctx, model, provider); err != nil {
    log.Printf("set model failed: %v", err)
    return fmt.Errorf("model %q/provider %q rejected: %w", model, provider, err)
}

Prevention

Validate model/provider IDs against the runtime's advertised list
Only pass AutoTier when the provider supports it
Upgrade SDK and runtime together when new models ship

When it happens

Trigger: Calling SetModel (or the session model API) with a model/provider identifier the runtime rejects, an unsupported AutoTier value for that provider, or when the RPC transport fails.

Common situations: Typo'd model ID; referencing a model removed in a newer runtime; passing a provider that doesn't serve the requested model; setting AutoTier on a provider without tier support.

Understand the failure class

Background: "API request failed": what wrapped HTTP errors from external APIs mean and how to find the real cause — this error's family across 29 libraries.

Related errors

AI-assisted analysis of github/copilot-sdk@cd8cf15dc3 (2026-09-09). Data as JSON: /api/errors/2971202e7fb06217. Report an issue: GitHub.

Appendix: source

Thrown at go/session.go:2006

		params.ReasoningSummary = opts.ReasoningSummary
		params.ContextTier = opts.ContextTier
		params.ModelCapabilities = opts.ModelCapabilities

		// The generated field is a double pointer so the three cases stay
		// distinct on the wire: a nil outer pointer omits the field and leaves
		// any staged preference alone, while a non-nil outer pointer sends the
		// inner value, including an explicit null.
		switch {
		case opts.AutoTier != nil:
			params.AutoTier = &opts.AutoTier
		case opts.ResetAutoTier:
			var providerDefault *AutoTier
			params.AutoTier = &providerDefault
		}
	}
	_, err := s.RPC.Model.SwitchTo(ctx, params)
	if err != nil {
		return fmt.Errorf("failed to set model: %w", err)
	}

	return nil
}

// SetAutoTier changes the Auto routing preference without changing the selected model.
//
// The runtime does not apply the preference immediately. It records the request and
// commits it only when a later user turn using the "auto" model successfully obtains a
// usable model from the provider. A [rpc.ModelSwitchAutoTierStatusPending] status
// therefore confirms that the request was accepted, not that it took effect.
//
// Watch for the outcome through the session.model_change event on success, or the
// ephemeral session.auto_tier_switch_failed event on failure. You can also read the
// current committed and in-flight state at any time with session.RPC.Model.GetCurrent.
//
// Only the most recent request survives: issuing a new request replaces any earlier one
// that has not yet been claimed by a turn.

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