What causes "discover tools: %w" in micro/go-micro?

Inspect the wrapped error (%w) to find the underlying discovery failure

discover tools: %w

Error message

discover tools: %w

What it means

This error wraps any failure from agent.discoverTools() during the Stream path (agent/agent.go:272). discoverTools calls tools.Discover() on the configured tool source to enumerate the tool list for a run; if that discovery fails (registry unreachable, bad service config, backend error), the agent cannot proceed and the underlying error is surfaced with the 'discover tools:' prefix.

Source

Thrown at agent/agent.go:272

func (a *agentImpl) Ask(ctx context.Context, message string) (*Response, error) {
	return a.ask(ctx, message, a.parentRunID)
}

// Stream sends a message and returns a streaming model response. Tool-calling
// agent runs still use Ask; Stream is for chat turns where immediate token
// delivery is more important than tool orchestration.
func (a *agentImpl) Stream(ctx context.Context, message string) (ai.Stream, error) {
	a.mu.Lock()
	defer a.mu.Unlock()
	if err := ctx.Err(); err != nil {
		return nil, err
	}
	if a.model == nil {
		a.setup()
	}
	toolList, err := a.discoverTools()
	if err != nil {
		return nil, fmt.Errorf("discover tools: %w", err)
	}
	runID := uuid.New().String()
	ctx = ai.WithRunInfo(ctx, ai.RunInfo{
		RunID:    runID,
		ParentID: a.parentRunID,
		Agent:    a.opts.Name,
	})
	// Messages carries the history; Prompt carries the turn being answered.
	// Providers build their payload as Messages followed by Prompt, so
	// appending the current message here would send it to the model twice.
	// Memory has not recorded this turn yet (that happens after the stream
	// starts), so the copy is passed through untrimmed: a trailing user
	// message equal to this one is real prior context, not a duplicate.
	messages := append([]ai.Message(nil), a.mem.Messages()...)
	stream, err := a.model.Stream(ctx, &ai.Request{
		Prompt:       message,
		SystemPrompt: a.buildPrompt(),
		Tools:        toolList,

View on GitHub (pinned to 24529f1404)

Solutions

Inspect the wrapped error (%w) to find the underlying discovery failure
Verify the tool registry/service addresses configured via agent Options are reachable (curl/ping the endpoint)
Confirm the tool service is started and registered before calling Stream
Re-check tool service credentials/auth and API version compatibility
Retry after fixing; discovery is attempted per-run so no agent restart is needed

Example fix

// before
resp, err := agent.Stream(ctx, msg)
// ignore wrapped discovery cause
// after
resp, err := agent.Stream(ctx, msg)
if err != nil && strings.Contains(err.Error(), "discover tools") {
    log.Printf("tool discovery failed: %v", errors.Unwrap(err))
    // check tool service health before retrying
}

Defensive patterns

Strategy: try-catch

Validate before calling

// pre-flight: verify the tool source can be discovered
if h, ok := toolSource.(interface{ HealthCheck(context.Context) error }); ok {
    if err := h.HealthCheck(ctx); err != nil {
        return fmt.Errorf("tool service not ready: %w", err)
    }
}

Type guard

func isDiscoverToolsErr(err error) bool {
    return err != nil && strings.Contains(err.Error(), "discover tools:")
}

Try / catch

resp, err := ag.Stream(ctx, msg)
if err != nil {
    var base error
    if strings.HasPrefix(err.Error(), "discover tools:") && errors.As(err, &base) {
        return fmt.Errorf("tool discovery failed, check tool service: %w", base)
    }
    return err
}

Prevention

Health-check the tool registry before starting runs
Pin and test tool service endpoints in configuration
Alert on tool service liveness in the same deployment as the agent
Retry transient discovery failures with backoff

When it happens

Trigger: Calling Stream (or the run-index commands built on it, e.g. printRunIndex) when a.tools.Discover() returns an error: the tool registry/service is down, a configured service fails to respond, or tool discovery returns a malformed result.

Common situations: Misconfigured tool service endpoints in agent Options, the tool registry microservice not running, network/auth failures reaching a remote tool provider, or a version mismatch where the tool service returns an incompatible response.

Related errors

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/dc7c9a0add467fc5. Report an issue: GitHub.