diff --git a/src/cli/operations/dev/__tests__/codezip-dev-server.test.ts b/src/cli/operations/dev/__tests__/codezip-dev-server.test.ts
index f0e0a2fe1..176aeb1d2 100644
--- a/src/cli/operations/dev/__tests__/codezip-dev-server.test.ts
+++ b/src/cli/operations/dev/__tests__/codezip-dev-server.test.ts
@@ -127,6 +127,8 @@ describe('CodeZipDevServer spawn config', () => {
     expect(env.AGENTCORE_RUNTIME_URL).toBe('http://localhost:8080/');
     expect(env.LOCAL_DEV).toBe('1');
     expect(env.MY_KEY).toBe('secret');
+    // serve_a2a() reads A2A_PORT, not PORT, so the dev port must reach it here.
+    expect(env.A2A_PORT).toBe('8080');
   });
 
   it('TypeScript HTTP: uses npx tsx watch with the entry file', async () => {
diff --git a/src/cli/operations/dev/__tests__/container-dev-server.test.ts b/src/cli/operations/dev/__tests__/container-dev-server.test.ts
index babb19b98..57772446b 100644
--- a/src/cli/operations/dev/__tests__/container-dev-server.test.ts
+++ b/src/cli/operations/dev/__tests__/container-dev-server.test.ts
@@ -355,6 +355,18 @@ describe('ContainerDevServer', () => {
       expect(spawnArgs).toContain('9001:9000');
       expect(spawnArgs).toContain('PORT=9000');
       expect(spawnArgs).toContain('AGENTCORE_RUNTIME_URL=http://localhost:9001/');
+      // serve_a2a() reads A2A_PORT, not PORT.
+      expect(spawnArgs).toContain('A2A_PORT=9000');
+    });
+
+    it('does not set A2A_PORT for non-A2A protocols', async () => {
+      mockSuccessfulPrepare();
+
+      const server = new ContainerDevServer(defaultConfig, defaultOptions);
+      await server.start();
+
+      const spawnArgs = getSpawnArgs();
+      expect(spawnArgs.some((a: string) => a.startsWith('A2A_PORT='))).toBe(false);
     });
 
     it('maps an MCP host port to the MCP container port', async () => {
diff --git a/src/cli/operations/dev/codezip-dev-server.ts b/src/cli/operations/dev/codezip-dev-server.ts
index 3a6f9f98a..76047749c 100644
--- a/src/cli/operations/dev/codezip-dev-server.ts
+++ b/src/cli/operations/dev/codezip-dev-server.ts
@@ -1,5 +1,6 @@
 import { getVenvExecutable } from '../../../lib/utils/platform';
 import type { ProtocolMode } from '../../../schema';
+import { A2A_PORT_ENV } from './constants';
 import { DevServer, type LogLevel, type SpawnConfig } from './dev-server';
 import { convertEntrypointToModule } from './utils';
 import { spawnSync } from 'child_process';
@@ -147,6 +148,8 @@ export class CodeZipDevServer extends DevServer {
     }
     if (protocol === 'A2A') {
       env.AGENTCORE_RUNTIME_URL = `http://localhost:${port}/`;
+      // serve_a2a() reads A2A_PORT, not PORT, so the host-side dev port reaches it.
+      env[A2A_PORT_ENV] = String(port);
     }
 
     if (!isPython) {
diff --git a/src/cli/operations/dev/constants.ts b/src/cli/operations/dev/constants.ts
index 1519da92a..70745d3a9 100644
--- a/src/cli/operations/dev/constants.ts
+++ b/src/cli/operations/dev/constants.ts
@@ -1,2 +1,9 @@
 export const MCP_DEFAULT_PORT = 8000;
 export const A2A_DEFAULT_PORT = 9000;
+
+/**
+ * Protocol-scoped port override read by the SDK's `serve_a2a()`. The generic
+ * `PORT` is not used for A2A: shared images set it to another protocol's port
+ * (8080 for HTTP, 8000 for MCP), which would bind the A2A server off-contract.
+ */
+export const A2A_PORT_ENV = 'A2A_PORT';
diff --git a/src/cli/operations/dev/container-dev-server.ts b/src/cli/operations/dev/container-dev-server.ts
index d53c7e372..647646a8e 100644
--- a/src/cli/operations/dev/container-dev-server.ts
+++ b/src/cli/operations/dev/container-dev-server.ts
@@ -2,7 +2,7 @@ import { CONTAINER_INTERNAL_PORT, DOCKERFILE_NAME, getDockerfilePath } from '../
 import { getCustomBuildArgs, getUvBuildArgs } from '../../../lib/packaging/build-args';
 import { ensureBuildContextDockerignore } from '../../../lib/packaging/build-context-dockerignore';
 import { detectContainerRuntime } from '../../external-requirements/detect';
-import { A2A_DEFAULT_PORT, MCP_DEFAULT_PORT } from './constants';
+import { A2A_DEFAULT_PORT, A2A_PORT_ENV, MCP_DEFAULT_PORT } from './constants';
 import { DevServer, type LogLevel, type SpawnConfig } from './dev-server';
 import { waitForServerReady } from './utils';
 import { type ChildProcess, spawn, spawnSync } from 'child_process';
@@ -250,7 +250,9 @@ export class ContainerDevServer extends DevServer {
       ...containerEnvVars,
       LOCAL_DEV: '1',
       PORT: String(internalPort),
-      ...(this.config.protocol === 'A2A' ? { AGENTCORE_RUNTIME_URL: `http://localhost:${port}/` } : {}),
+      ...(this.config.protocol === 'A2A'
+        ? { AGENTCORE_RUNTIME_URL: `http://localhost:${port}/`, [A2A_PORT_ENV]: String(internalPort) }
+        : {}),
     }).flatMap(([k, v]) => ['-e', `${k}=${v}`]);
 
     return {