diff --git a/packages/happy-cli/src/claude/utils/OutgoingMessageQueue.test.ts b/packages/happy-cli/src/claude/utils/OutgoingMessageQueue.test.ts
new file mode 100644
index 000000000..f2ce0e3f8
--- /dev/null
+++ b/packages/happy-cli/src/claude/utils/OutgoingMessageQueue.test.ts
@@ -0,0 +1,126 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { OutgoingMessageQueue } from './OutgoingMessageQueue';
+
+describe('OutgoingMessageQueue', () => {
+    let sentMessages: any[];
+    let queue: OutgoingMessageQueue;
+
+    beforeEach(() => {
+        sentMessages = [];
+        queue = new OutgoingMessageQueue((msg) => sentMessages.push(msg));
+    });
+
+    afterEach(() => {
+        queue.destroy();
+    });
+
+    // Small delay to let the async lock chain and setTimeout(0) scheduling resolve
+    const tick = (ms = 50) => new Promise(r => setTimeout(r, ms));
+
+    it('should send non-delayed messages immediately', async () => {
+        queue.enqueue({ type: 'text', content: 'hello' });
+        await tick();
+
+        expect(sentMessages).toHaveLength(1);
+        expect(sentMessages[0].content).toBe('hello');
+    });
+
+    it('should send multiple non-delayed messages in order', async () => {
+        queue.enqueue({ type: 'text', content: 'first' });
+        queue.enqueue({ type: 'text', content: 'second' });
+        queue.enqueue({ type: 'text', content: 'third' });
+        await tick();
+
+        expect(sentMessages).toHaveLength(3);
+        expect(sentMessages[0].content).toBe('first');
+        expect(sentMessages[1].content).toBe('second');
+        expect(sentMessages[2].content).toBe('third');
+    });
+
+    it('should delay messages with delay option', async () => {
+        queue.enqueue({ type: 'text', content: 'delayed' }, { delay: 100 });
+        await tick();
+        expect(sentMessages).toHaveLength(0);
+
+        // Wait for delay to expire
+        await tick(150);
+
+        expect(sentMessages).toHaveLength(1);
+        expect(sentMessages[0].content).toBe('delayed');
+    });
+
+    it('should NOT block released messages behind unreleased ones', async () => {
+        // This is the core fix for head-of-line blocking (#639)
+        queue.enqueue({ type: 'text', content: 'delayed-tool-call' }, {
+            delay: 200,
+            toolCallIds: ['tool-1']
+        });
+        queue.enqueue({ type: 'text', content: 'immediate-result' });
+        await tick();
+
+        // The immediate message should be sent even though delayed one is in queue
+        expect(sentMessages).toHaveLength(1);
+        expect(sentMessages[0].content).toBe('immediate-result');
+
+        // After delay expires, the delayed message should also be sent
+        await tick(250);
+
+        expect(sentMessages).toHaveLength(2);
+        expect(sentMessages[1].content).toBe('delayed-tool-call');
+    });
+
+    it('should release delayed messages via releaseToolCall', async () => {
+        queue.enqueue({ type: 'text', content: 'tool-call-msg' }, {
+            delay: 500,
+            toolCallIds: ['tool-1']
+        });
+        await tick();
+        expect(sentMessages).toHaveLength(0);
+
+        // Release via tool call ID (before delay expires)
+        await queue.releaseToolCall('tool-1');
+        await tick();
+
+        expect(sentMessages).toHaveLength(1);
+        expect(sentMessages[0].content).toBe('tool-call-msg');
+    });
+
+    it('should not send system type messages', async () => {
+        queue.enqueue({ type: 'system', content: 'internal' });
+        await tick();
+
+        expect(sentMessages).toHaveLength(0);
+    });
+
+    it('should flush all messages immediately', async () => {
+        queue.enqueue({ type: 'text', content: 'delayed1' }, { delay: 500 });
+        queue.enqueue({ type: 'text', content: 'delayed2' }, { delay: 500 });
+        queue.enqueue({ type: 'text', content: 'immediate' });
+        await tick();
+
+        // Only immediate should have been sent (delayed ones skipped)
+        expect(sentMessages).toHaveLength(1);
+        expect(sentMessages[0].content).toBe('immediate');
+
+        await queue.flush();
+        await tick();
+
+        expect(sentMessages).toHaveLength(3);
+    });
+
+    it('should handle interleaved delayed and immediate messages', async () => {
+        queue.enqueue({ type: 'text', content: 'tool-call-1' }, { delay: 200, toolCallIds: ['t1'] });
+        queue.enqueue({ type: 'text', content: 'sidechain-result' });
+        queue.enqueue({ type: 'text', content: 'tool-call-2' }, { delay: 200, toolCallIds: ['t2'] });
+        await tick();
+
+        // The sidechain result should be sent immediately
+        expect(sentMessages).toHaveLength(1);
+        expect(sentMessages[0].content).toBe('sidechain-result');
+
+        // After 200ms both tool calls should be released
+        await tick(250);
+
+        expect(sentMessages).toHaveLength(3);
+    });
+});
diff --git a/packages/happy-cli/src/claude/utils/OutgoingMessageQueue.ts b/packages/happy-cli/src/claude/utils/OutgoingMessageQueue.ts
index 6d214c657..f8ae9b7e4 100644
--- a/packages/happy-cli/src/claude/utils/OutgoingMessageQueue.ts
+++ b/packages/happy-cli/src/claude/utils/OutgoingMessageQueue.ts
@@ -103,22 +103,25 @@ export class OutgoingMessageQueue {
     }
     
     /**
-     * Process queue - send messages in ID order that are released
+     * Process queue - send released messages in ID order, skipping
+     * unreleased items so they don't block later ready messages.
      * (Internal implementation without lock)
      */
     private processQueueInternal(): void {
         // Sort by ID to ensure order
         this.queue.sort((a, b) => a.id - b.id);
-        
-        // Process from front of queue
-        while (this.queue.length > 0) {
-            const item = this.queue[0];
-            
-            // If not released yet, stop processing (maintain order)
+
+        // Send all released items, skipping unreleased ones
+        let i = 0;
+        while (i < this.queue.length) {
+            const item = this.queue[i];
+
+            // Skip unreleased items — they'll be sent when released
             if (!item.released) {
-                break;
+                i++;
+                continue;
             }
-            
+
             // Send if not already sent
             if (!item.sent) {
                 if (item.logMessage.type !== 'system') {
@@ -126,9 +129,10 @@ export class OutgoingMessageQueue {
                 }
                 item.sent = true;
             }
-            
-            // Remove from queue
-            this.queue.shift();
+
+            // Remove sent item from queue
+            this.queue.splice(i, 1);
+            // Don't increment i since splice shifts remaining items
         }
     }