fix: filter non-JSON log messages from ACP agent output

src/client.ts

@@ -967,7 +967,8 @@ export class AcpClient {
 
     const input = Writable.toWeb(child.stdin);
     const output = Readable.toWeb(child.stdout) as ReadableStream<Uint8Array>;
-    const stream = this.createTappedStream(ndJsonStream(input, output));
+    const filteredOutput = this.filterAcpOutputStream(output);
+    const stream = this.createTappedStream(ndJsonStream(input, filteredOutput));
 
     const connection = new ClientSideConnection(
       () => ({
@@ -1066,6 +1067,19 @@ export class AcpClient {
       return this.suppressReplaySessionUpdateMessages && isSessionUpdateNotification(message);
     };
 
+    const isValidAcpMessage = (value: AnyMessage): boolean => {
+      if (!value || typeof value !== "object") {
+        return false;
+      }
+      return (
+        "jsonrpc" in value ||
+        "method" in value ||
+        "id" in value ||
+        "result" in value ||
+        "error" in value
+      );
+    };
+
     const readable = new ReadableStream<AnyMessage>({
       async start(controller) {
         const reader = base.readable.getReader();
@@ -1078,6 +1092,9 @@ export class AcpClient {
             if (!value) {
               continue;
             }
+            if (!isValidAcpMessage(value)) {
+              continue;
+            }
             if (!shouldSuppressInboundReplaySessionUpdate(value)) {
               onAcpOutputMessage()?.("inbound", value);
               onAcpMessage()?.("inbound", value);
@@ -1674,4 +1691,77 @@ export class AcpClient {
 
     throw new Error(`Timed out waiting for session replay drain after ${normalizedTimeoutMs}ms`);
   }
+
+  private filterAcpOutputStream(output: ReadableStream<Uint8Array>): ReadableStream<Uint8Array> {
+    const textDecoder = new TextDecoder();
+    const textEncoder = new TextEncoder();
+    let buffer = "";
+    let nonJsonLineCount = 0;
+    const maxNonJsonLines = 10;
+
+    return new ReadableStream<Uint8Array>({
+      async start(controller) {
+        const reader = output.getReader();
+        try {
+          while (true) {
+            const { value, done } = await reader.read();
+            if (done) {
+              // Flush any remaining buffered content at EOF
+              if (buffer.trim().length > 0) {
+                try {
+                  JSON.parse(buffer.trim());
+                  controller.enqueue(textEncoder.encode(buffer + "\n"));
+                } catch {
+                  nonJsonLineCount += 1;
+                  if (nonJsonLineCount > maxNonJsonLines) {
+                    throw new Error(
+                      `Agent stdout exceeded ${maxNonJsonLines} non-JSON lines without completing ACP handshake. ` +
+                        "This indicates the adapter does not support stdio ACP mode.",
+                    );
+                  }
+                }
+              }
+              break;
+            }
+            if (!value) {
+              continue;
+            }
+
+            buffer += textDecoder.decode(value, { stream: true });
+            const lines = buffer.split("\n");
+            buffer = lines.pop() || "";
+
+            for (const line of lines) {
+              const trimmedLine = line.trim();
+              if (!trimmedLine) {
+                continue;
+              }
+
+              // Try to parse as JSON to check if it's a valid ACP message
+              try {
+                JSON.parse(trimmedLine);
+                // If it parses successfully, it's valid JSON - pass it through
+                const outputLine = trimmedLine + "\n";
+                controller.enqueue(textEncoder.encode(outputLine));
+                nonJsonLineCount = 0;
+              } catch {
+                // If it fails to parse, it's likely a log message - skip it silently
+                // This prevents "[iFlow ACP Agent] ..." messages from causing parse errors
+                nonJsonLineCount += 1;
+                if (nonJsonLineCount > maxNonJsonLines) {
+                  throw new Error(
+                    `Agent stdout exceeded ${maxNonJsonLines} non-JSON lines without completing ACP handshake. ` +
+                      "This indicates the adapter does not support stdio ACP mode.",
+                  );
+                }
+              }
+            }
+          }
+        } finally {
+          reader.releaseLock();
+          controller.close();
+        }
+      },
+    });
+  }
 }