diff --git a/src/lib/server/create-adcp-server.ts b/src/lib/server/create-adcp-server.ts
index 1ca16104b..2420be21e 100644
--- a/src/lib/server/create-adcp-server.ts
+++ b/src/lib/server/create-adcp-server.ts
@@ -2729,6 +2729,7 @@ function toProtocolTaskStatus(task: TaskRecord): GetTaskStatusResponse | undefin
       ? { completed_at: task.updatedAt }
       : {}),
     ...(task.hasWebhook !== undefined ? { has_webhook: task.hasWebhook === true } : {}),
+    ...(task.ext !== undefined ? { ext: task.ext } : {}),
     ...(progress !== undefined ? { progress } : {}),
     ...(task.status === 'rejected' && task.statusMessage !== undefined ? { message: task.statusMessage } : {}),
     ...(task.error !== undefined
@@ -2762,6 +2763,8 @@ function toProtocolTaskListItem(task: TaskRecord): ListTasksResponse['tasks'][nu
       ? { completed_at: task.updatedAt }
       : {}),
     ...(task.hasWebhook !== undefined ? { has_webhook: task.hasWebhook === true } : {}),
+    // The list item schema is open (passthrough); `ext` is the spec's vendor slot.
+    ...(task.ext !== undefined ? ({ ext: task.ext } as Record<string, unknown>) : {}),
   };
 }
 
diff --git a/src/lib/server/decisioning/async-outcome.ts b/src/lib/server/decisioning/async-outcome.ts
index aa17005b4..da5b4cce4 100644
--- a/src/lib/server/decisioning/async-outcome.ts
+++ b/src/lib/server/decisioning/async-outcome.ts
@@ -221,6 +221,14 @@ export interface TaskHandoffOptions {
    * The caller is responsible for uniqueness within the account.
    */
   task_id?: string;
+  /**
+   * Vendor-namespaced extension object echoed on the submitted envelope
+   * (`{ status: 'submitted', task_id, ext }`). The spec's submitted branch
+   * declares `ext`; keys MUST be namespaced under a vendor key (`ext.acme`),
+   * never a spec field such as `media_buy_id` — those belong on the terminal
+   * artifact. Not persisted; a registry decorator may re-attach it on reads.
+   */
+  ext?: Record<string, unknown>;
 }
 
 /**
diff --git a/src/lib/server/decisioning/runtime/from-platform.ts b/src/lib/server/decisioning/runtime/from-platform.ts
index eba8be755..ee615bf92 100644
--- a/src/lib/server/decisioning/runtime/from-platform.ts
+++ b/src/lib/server/decisioning/runtime/from-platform.ts
@@ -4089,6 +4089,7 @@ function buildTasksGetTool<P extends DecisioningPlatform<any, any>>(
           logger.warn?.('Omitting unsafe stored task progress during tasks_get poll');
         }
       }
+      if (record.ext !== undefined) payload.ext = record.ext;
       if (
         args.include_result === true &&
         (record.status === 'completed' || record.status === 'failed' || record.status === 'rejected') &&
@@ -4441,6 +4442,8 @@ function buildDefaultTaskRegistry(): TaskRegistry {
 type SubmittedEnvelope = {
   status: 'submitted';
   task_id: string;
+  /** Adopter-supplied, vendor-namespaced extension (`TaskHandoffOptions.ext`). */
+  ext?: Record<string, unknown>;
 };
 
 /**
@@ -4812,7 +4815,8 @@ async function routeIfHandoff<TInner, TWire>(
           await externalTaskFn(buildExternalHandoffContext(taskRegistry, taskRef, opts.servedAdcpVersion));
         },
         options.task_id,
-        'external'
+        'external',
+        options.ext
       );
     }
     let handoffTaskStarted = false;
@@ -4840,7 +4844,9 @@ async function routeIfHandoff<TInner, TWire>(
           await lifecycle?.onHandoffSuccess?.(inner);
           return await project(inner);
         },
-        options?.task_id
+        options?.task_id,
+        'framework',
+        options?.ext
       );
     } catch (error) {
       // Allocation/registration failures happen before the background task
@@ -4860,7 +4866,8 @@ async function dispatchHitl<TResult>(
   opts: DispatchHitlOpts,
   taskFn: (taskRef: ScopedTaskRef) => Promise<TResult>,
   overrideTaskId?: string,
-  settlement: 'framework' | 'external' = 'framework'
+  settlement: 'framework' | 'external' = 'framework',
+  ext?: Record<string, unknown>
 ): Promise<SubmittedEnvelope> {
   // Fail before task creation, external producer callbacks, or any terminal
   // state can be persisted. A buyer gets Submitted only after a validated
@@ -5088,7 +5095,7 @@ async function dispatchHitl<TResult>(
   })();
   taskRegistry._registerBackground(taskId, taskRef, completion);
 
-  return { status: 'submitted', task_id: taskId };
+  return { status: 'submitted', task_id: taskId, ...(ext !== undefined && { ext }) };
 }
 
 /**
diff --git a/src/lib/server/decisioning/runtime/task-registry.ts b/src/lib/server/decisioning/runtime/task-registry.ts
index 66da046d8..6e6683164 100644
--- a/src/lib/server/decisioning/runtime/task-registry.ts
+++ b/src/lib/server/decisioning/runtime/task-registry.ts
@@ -144,6 +144,13 @@ export interface TaskRecord<TResult = unknown, TError extends AdcpStructuredErro
    * `tasks_get` via the spec `progress` field.
    */
   progress?: TaskHandoffProgress;
+  /**
+   * Vendor-namespaced extension object projected as `ext` on task reads
+   * (`get_task_status`, `tasks_get`, `list_tasks` items). The built-in
+   * registries never persist it; a decorating registry attaches it at read
+   * time (e.g. the vendor-side object a task holds).
+   */
+  ext?: Record<string, unknown>;
   /**
    * Whether the buyer wired `push_notification_config.url` and the dispatch
    * had an emitter capable of delivering it. Surfaced to the buyer via
diff --git a/test/server-create-adcp-server.test.js b/test/server-create-adcp-server.test.js
index 3a69c3012..4bf85754f 100644
--- a/test/server-create-adcp-server.test.js
+++ b/test/server-create-adcp-server.test.js
@@ -2989,6 +2989,39 @@ describe('createAdcpServer', () => {
       assert.strictEqual(listed.structuredContent.adcp_error.code, 'VALIDATION_ERROR');
     });
 
+    it('projects a decorating registry ext onto get_task_status and list_tasks items', async () => {
+      const inner = createInMemoryTaskRegistry();
+      const held = await inner.create({ tool: 'create_media_buy', accountId: 'acct_1', ownerScope: 'api_key:buyer-1' });
+      const ext = { acme: { media_buy_id: 'mb_42', campaign_link: 'https://acme.example/c/42' } };
+      // A vendor decorator attaches the extension at read time; the built-in registry never stores it.
+      const taskRegistry = {
+        ...inner,
+        getTask: async (taskId, scope) => {
+          const record = await inner.getTask(taskId, scope);
+          return record && record.taskId === held.taskId ? { ...record, ext } : record;
+        },
+        list: async opts => {
+          const listed = await inner.list(opts);
+          return { ...listed, tasks: listed.tasks.map(t => (t.taskId === held.taskId ? { ...t, ext } : t)) };
+        },
+      };
+      const server = createAdcpServer({
+        name: 'Test',
+        version: '1.0.0',
+        taskRegistry,
+        resolveAccountFromAuth: async () => ({ id: 'acct_1' }),
+      });
+      const buyerOne = { authInfo: { credential: { kind: 'api_key', key_id: 'buyer-1' } } };
+
+      const status = await callTool(server, 'get_task_status', { task_id: held.taskId }, buyerOne);
+      assert.strictEqual(status.status, 'submitted');
+      assert.deepStrictEqual(status.ext, ext);
+
+      const listed = await callTool(server, 'list_tasks', {}, buyerOne);
+      assert.strictEqual(listed.tasks.length, 1);
+      assert.deepStrictEqual(listed.tasks[0].ext, ext);
+    });
+
     it('answers get_task_status/list_tasks from the scoped AdCP task registry only', async () => {
       const taskRegistry = createInMemoryTaskRegistry();
       const owned = await taskRegistry.create({
diff --git a/test/server-decisioning-mock-seller.test.js b/test/server-decisioning-mock-seller.test.js
index b7ea550a7..1a207d5cb 100644
--- a/test/server-decisioning-mock-seller.test.js
+++ b/test/server-decisioning-mock-seller.test.js
@@ -119,27 +119,30 @@ function makeSyncMockSeller({ floorCpm = 1.0 } = {}) {
   return platform;
 }
 
-function makeHitlMockSeller({ floorCpm = 1.0, approvalDurationMs = 30 } = {}) {
+function makeHitlMockSeller({ floorCpm = 1.0, approvalDurationMs = 30, ext } = {}) {
   const platform = basePlatformShape({
     createMediaBuy: (req, ctx) =>
-      ctx.handoffToTask(async () => {
-        const errors = preflight(req, { floorCpm });
-        if (errors.length > 0) {
-          throw new AdcpError('INVALID_REQUEST', {
-            recovery: 'correctable',
-            message: errors[0].message,
-            field: errors[0].field,
-            details: { errors },
-          });
-        }
-        // Trafficker review window
-        await new Promise(r => setTimeout(r, approvalDurationMs));
-        const buyId = `mb_${Date.now()}_${Math.floor(Math.random() * 1000)}`;
-        const totalBudget = typeof req.total_budget === 'number' ? req.total_budget : (req.total_budget?.amount ?? 0);
-        const buy = { media_buy_id: buyId, status: 'active', total_budget: totalBudget };
-        platform.mediaBuys.set(buyId, buy);
-        return buy;
-      }),
+      ctx.handoffToTask(
+        async () => {
+          const errors = preflight(req, { floorCpm });
+          if (errors.length > 0) {
+            throw new AdcpError('INVALID_REQUEST', {
+              recovery: 'correctable',
+              message: errors[0].message,
+              field: errors[0].field,
+              details: { errors },
+            });
+          }
+          // Trafficker review window
+          await new Promise(r => setTimeout(r, approvalDurationMs));
+          const buyId = `mb_${Date.now()}_${Math.floor(Math.random() * 1000)}`;
+          const totalBudget = typeof req.total_budget === 'number' ? req.total_budget : (req.total_budget?.amount ?? 0);
+          const buy = { media_buy_id: buyId, status: 'active', total_budget: totalBudget };
+          platform.mediaBuys.set(buyId, buy);
+          return buy;
+        },
+        ext !== undefined ? { ext } : undefined
+      ),
   });
   return platform;
 }
@@ -226,6 +229,30 @@ describe('MockSeller worked example — unified hybrid shape', () => {
       assert.strictEqual(final.result.status, 'active');
     });
 
+    it('echoes the adopter ext on the submitted envelope, vendor-namespaced', async () => {
+      const ext = { acme: { media_buy_id: 'mb_pending_1', campaign_link: 'https://acme.example/c/1' } };
+      const platform = makeHitlMockSeller({ approvalDurationMs: 30, ext });
+      const server = buildServer(platform);
+      const result = await dispatchCreate(server, { total_budget: 100_000 });
+
+      assert.strictEqual(result.structuredContent.status, 'submitted');
+      assert.deepStrictEqual(result.structuredContent.ext, ext);
+      assert.strictEqual(result.structuredContent.media_buy_id, undefined);
+
+      await server.awaitTaskUnsafe(result.structuredContent.task_id);
+    });
+
+    it('omits ext from the submitted envelope when the adopter passed none', async () => {
+      const platform = makeHitlMockSeller({ approvalDurationMs: 30 });
+      const server = buildServer(platform);
+      const result = await dispatchCreate(server, { total_budget: 100_000 });
+
+      assert.strictEqual(result.structuredContent.status, 'submitted');
+      assert.ok(!('ext' in result.structuredContent));
+
+      await server.awaitTaskUnsafe(result.structuredContent.task_id);
+    });
+
     it('background AdcpError records terminal failed with structured fields', async () => {
       const platform = makeHitlMockSeller();
       const server = buildServer(platform);