github/copilot-sdk · error

failed to unmarshal schema for type

Error message

failed to unmarshal schema for type %v: %v

What it means

generateSchemaForType round-trips the marshaled schema through json.Unmarshal into map[string]any before returning it. If the JSON just produced by json.Marshal cannot be unmarshaled into a map, it panics. In practice this is an internal invariant: freshly marshaled schema JSON should always decode, so this points at corrupted marshaling or memory issues.

Solutions

1. Verify the schema marshals to a JSON object (not an array/scalar) — top-level must decode into map[string]any
2. Rebuild without vendored/patched encoding/json
3. Inspect schemaBytes content if you modified the function
4. Report upstream if reproducible with an unmodified library

Example fix

// before
var schemaMap map[string]any
json.Unmarshal(schemaBytes, &schemaMap)
// after — ensure top-level is an object
var probe any
json.Unmarshal(schemaBytes, &probe)
if _, ok := probe.(map[string]any); !ok { /* ForType returned non-object schema */ }

var probe any
if err := json.Unmarshal(schemaBytes, &probe); err != nil {
	return fmt.Errorf("schema JSON invalid: %w", err)
}
if _, ok := probe.(map[string]any); !ok {
	return fmt.Errorf("schema is not a JSON object")
}

func isJSONObject(b []byte) bool {
	var m map[string]any
	return json.Unmarshal(b, &m) == nil && m != nil
}

defer func() {
	if r := recover(); r != nil {
		log.Printf("schema conversion failed: %v", r)
	}
}()

When it happens

Trigger: json.Unmarshal returning an error on schemaBytes that were marshaled in the immediately preceding step — practically only when marshaling produced invalid/large/truncated JSON or an environment-level fault; also seen in altered builds where schemaBytes was substituted.

Common situations: Build/toolchain anomalies, monkey-patched or vendored encoding/json, or downstream code replacing this function with one that marshals differently.

Understand the failure class

Background: "failed to unmarshal" / json.Unmarshal errors: why parsing a response into a Go struct fails and how to fix it — this error's family across 23 libraries.

Related errors

• failed to unmarshal models response
• failed to unmarshal get events response
• failed to decode session detach response
• failed to marshal schema for type
• failed to unmarshal response

AI-assisted analysis of github/copilot-sdk@cd8cf15dc3 (2026-09-09). Data as JSON: /api/errors/76a7e394b6ffc7f3. Report an issue: GitHub.

Appendix: source

Thrown at go/definetool.go:228

	if t.Kind() == reflect.Pointer {
		t = t.Elem()
	}

	// Use google/jsonschema-go to generate the schema
	schema, err := jsonschema.ForType(t, nil)
	if err != nil {
		panic(fmt.Sprintf("failed to generate schema for type %v: %v", t, err))
	}

	// Convert schema to map[string]any
	schemaBytes, err := json.Marshal(schema)
	if err != nil {
		panic(fmt.Sprintf("failed to marshal schema for type %v: %v", t, err))
	}

	var schemaMap map[string]any
	if err := json.Unmarshal(schemaBytes, &schemaMap); err != nil {
		panic(fmt.Sprintf("failed to unmarshal schema for type %v: %v", t, err))
	}

	return schemaMap
}

View on GitHub (pinned to cd8cf15dc3)