charmbracelet/crush · error

failed to decode MCP prompts: %w

Error message

failed to decode MCP prompts: %w

What it means

On a 200 response, ListMCPPrompts decodes the body into []proto.MCPPrompt. 'failed to decode MCP prompts: %w' means the JSON body does not match the expected prompt schema — typically a client/server version mismatch or an intermediary returning non-JSON content.

Source

Thrown at internal/client/config.go:335

	var contents []MCPResourceContents
	if err := json.NewDecoder(rsp.Body).Decode(&contents); err != nil {
		return nil, fmt.Errorf("failed to decode MCP resource: %w", err)
	}
	return contents, nil
}

func (c *Client) ListMCPPrompts(ctx context.Context, id string) ([]proto.MCPPrompt, error) {
	rsp, err := c.get(ctx, fmt.Sprintf("/workspaces/%s/mcp/prompts", id), nil, nil)
	if err != nil {
		return nil, fmt.Errorf("failed to list MCP prompts: %w", err)
	}
	defer rsp.Body.Close()
	if rsp.StatusCode != http.StatusOK {
		return nil, fmt.Errorf("failed to list MCP prompts: status code %d", rsp.StatusCode)
	}
	var prompts []proto.MCPPrompt
	if err := json.NewDecoder(rsp.Body).Decode(&prompts); err != nil {
		return nil, fmt.Errorf("failed to decode MCP prompts: %w", err)
	}
	return prompts, nil
}

// GetMCPPrompt retrieves a prompt from a named MCP server.
func (c *Client) GetMCPPrompt(ctx context.Context, id, clientID, promptID string, args map[string]string) (string, error) {
	rsp, err := c.post(ctx, fmt.Sprintf("/workspaces/%s/mcp/get-prompt", id), nil, jsonBody(struct {
		ClientID string            `json:"client_id"`
		PromptID string            `json:"prompt_id"`
		Args     map[string]string `json:"args"`
	}{ClientID: clientID, PromptID: promptID, Args: args}), http.Header{"Content-Type": []string{"application/json"}})
	if err != nil {
		return "", fmt.Errorf("failed to get MCP prompt: %w", err)
	}
	defer rsp.Body.Close()
	if rsp.StatusCode != http.StatusOK {
		return "", fmt.Errorf("failed to get MCP prompt: status code %d", rsp.StatusCode)
	}

View on GitHub (pinned to 7944b8e522)

Solutions

1. Align client and server versions.
2. Inspect the raw body (curl the endpoint) to confirm what was returned.
3. Bypass/check proxies to rule out HTML injection.
4. Update the client's proto.MCPPrompt types if the schema legitimately changed.

Example fix

// before
prompts, err := client.ListMCPPrompts(ctx, id)
// after
prompts, err := client.ListMCPPrompts(ctx, id)
if err != nil && strings.Contains(err.Error(), "failed to decode MCP prompts") {
    return nil, fmt.Errorf("prompt schema mismatch — update the coder client to match server version: %w", err)
}

// check version compatibility once at startup
info, err := client.BuildInfo(ctx)
if err != nil {
    return fmt.Errorf("cannot verify server compatibility: %w", err)
}

func isPromptDecodeError(err error) bool {
    return err != nil && strings.Contains(err.Error(), "failed to decode MCP prompts")
}
// usage: if isPromptDecodeError(err) { /* version mismatch — do not retry blindly */ }

prompts, err := client.ListMCPPrompts(ctx, id)
if isPromptDecodeError(err) {
    // schema mismatch: update client types or server version; don't retry
    return nil, fmt.Errorf("prompt schema mismatch with server: %w", err)
}
if err != nil { return nil, err }

When it happens

Trigger: json.Decode error: schema drift in proto.MCPPrompt, HTML error page from a proxy, empty or truncated body despite 200.

Common situations: Upgraded Coder server with a changed prompt schema while the client is old; an ingress proxy returning HTML 200 pages; custom MCP servers emitting unexpected prompt fields is fine, but a wholly different body shape is not.

Understand the failure class

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• failed to decode MCP resource: %w
• failed to decode response: %w
• failed to decode MCP prompt response: %w
• failed to decode workspaces: %w
• failed to decode MCP pending auth: %w

AI-assisted analysis of charmbracelet/crush@7944b8e522 (2026-08-29). Data as JSON: /api/errors/57d9b700ae6ed4ec. Report an issue: GitHub.