feat: support MCP-as-CLI progress messages on stderr (#28109)

Author: Copilot

actions/setup/js/mcp_cli_bridge.cjs

@@ -732,6 +732,118 @@ function showToolHelp(serverName, toolName, tools) {
 // Response formatting
 // ---------------------------------------------------------------------------
 
+/**
+ * Extract JSON-RPC messages from a response body that may be:
+ * - A JSON object
+ * - A JSON string
+ * - Server-Sent Events (SSE) payload containing multiple `data:` lines
+ *
+ * @param {unknown} responseBody
+ * @returns {unknown[]}
+ */
+function extractJSONRPCMessages(responseBody) {
+  if (responseBody == null) {
+    return [];
+  }
+
+  if (Array.isArray(responseBody)) {
+    return responseBody;
+  }
+
+  if (typeof responseBody === "object") {
+    return [responseBody];
+  }
+
+  if (typeof responseBody !== "string") {
+    return [];
+  }
+
+  const trimmed = responseBody.trim();
+  if (!trimmed) {
+    return [];
+  }
+
+  try {
+    return [JSON.parse(trimmed)];
+  } catch {
+    // Fall through to SSE parsing.
+  }
+
+  /** @type {unknown[]} */
+  const messages = [];
+  for (const line of trimmed.split(/\r?\n/)) {
+    if (!line.startsWith("data:")) {
+      continue;
+    }
+    const payload = line.slice(5).trim();
+    if (!payload || payload === "[DONE]") {
+      continue;
+    }
+    try {
+      messages.push(JSON.parse(payload));
+    } catch {
+      // Ignore non-JSON SSE data lines.
+    }
+  }
+
+  return messages;
+}
+
+/**
+ * Render MCP progress notifications to stderr.
+ *
+ * @param {unknown[]} messages - Parsed JSON-RPC message stream
+ */
+function renderProgressMessages(messages) {
+  for (const message of messages) {
+    if (!message || typeof message !== "object" || !("method" in message) || message.method !== "notifications/progress") {
+      continue;
+    }
+
+    const params = "params" in message && message.params && typeof message.params === "object" ? message.params : null;
+    if (!params) {
+      continue;
+    }
+
+    const progressText = "message" in params && params.message ? String(params.message) : "";
+    const progress = "progress" in params && typeof params.progress === "number" ? params.progress : null;
+    const total = "total" in params && typeof params.total === "number" ? params.total : null;
+
+    if (progressText) {
+      process.stderr.write(progressText + "\n");
+      continue;
+    }
+
+    if (progress != null && total != null) {
+      process.stderr.write(`Progress: ${progress}/${total}\n`);
+      continue;
+    }
+
+    if (progress != null) {
+      process.stderr.write(`Progress: ${progress}\n`);
+      continue;
+    }
+
+    process.stderr.write(`Progress: ${JSON.stringify(params)}\n`);
+  }
+}
+
+/**
+ * @param {unknown} message
+ * @returns {boolean}
+ */
+function isErrorMessage(message) {
+  return !!(message && typeof message === "object" && "error" in message);
+}
+
+/**
+ * @param {unknown} message
+ * @returns {boolean}
+ */
+function isResultMessage(message) {
+  return !!(message && typeof message === "object" && "result" in message);
+}
+
 /**
  * Format and display the MCP tool call response.
  *
@@ -740,7 +852,10 @@ function showToolHelp(serverName, toolName, tools) {
  */
 function formatResponse(responseBody, serverName) {
   const core = global.core;
-  const resp = responseBody;
+  const messages = extractJSONRPCMessages(responseBody);
+  renderProgressMessages(messages);
+
+  const resp = messages.find(isErrorMessage) || messages.find(isResultMessage) || responseBody;
 
   // Check for JSON-RPC error
   if (resp && typeof resp === "object" && "error" in resp && resp.error && typeof resp.error === "object") {
@@ -905,6 +1020,8 @@ if (require.main === module) {
 module.exports = {
   parseToolArgs,
   coerceToolArgValue,
+  extractJSONRPCMessages,
+  renderProgressMessages,
   formatResponse,
   main,
 };

actions/setup/js/mcp_cli_bridge.test.cjs

@@ -187,4 +187,28 @@ describe("mcp_cli_bridge.cjs", () => {
     expect(stderrChunks.join("")).toContain("failed to audit workflow run");
     expect(process.exitCode).toBe(1);
   });
+
+  it("prints progress notifications to stderr and final text result to stdout for SSE responses", () => {
+    const sseBody = [
+      'data: {"jsonrpc":"2.0","method":"notifications/progress","params":{"progressToken":"abc","progress":1,"total":3,"message":"Step 1/3"}}',
+      'data: {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"done"}]}}',
+      "",
+    ].join("\n");
+
+    formatResponse(sseBody, "agenticworkflows");
+
+    expect(stderrChunks.join("")).toContain("Step 1/3");
+    expect(stdoutChunks.join("")).toBe("done\n");
+    expect(process.exitCode).toBe(0);
+  });
+
+  it("prints numeric progress to stderr when progress notification has no message", () => {
+    const sseBody = ['data: {"jsonrpc":"2.0","method":"notifications/progress","params":{"progressToken":"abc","progress":2,"total":5}}', 'data: {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"ok"}]}}', ""].join("\n");
+
+    formatResponse(sseBody, "agenticworkflows");
+
+    expect(stderrChunks.join("")).toContain("Progress: 2/5");
+    expect(stdoutChunks.join("")).toBe("ok\n");
+    expect(process.exitCode).toBe(0);
+  });
 });

pkg/cli/spec_test.go

@@ -1117,11 +1117,11 @@ func TestSpec_PublicAPI_ValidateWorkflowIntent(t *testing.T) {
 // Spec: "Sets a field in frontmatter YAML"
 func TestSpec_PublicAPI_UpdateFieldInFrontmatter(t *testing.T) {
 	tests := []struct {
-		name       string
-		content    string
-		fieldName  string
-		fieldValue string
-		wantErr    bool
+		name          string
+		content       string
+		fieldName     string
+		fieldValue    string
+		wantErr       bool
 		checkContains string
 	}{
 		{