feat(core): add subagent protocol enhancements

- AgentCapabilitiesSchema with supportsStreaming/Reasoning/Background flags
- supportsStreaming and supportsReasoningStream fields in ProviderSnapshotEntry
- new_task tool: background mode flag for non-blocking subtask dispatch

apps/coder/src/services/tools/new_task.ts

@@ -6,6 +6,7 @@ const NewTaskInput = z.object({
   input: z.string().min(1).describe('Task description for the child subtask'),
   agent: z.string().optional().describe('Optional: dispatch to a specific agent'),
   model: z.string().optional().describe('Optional: model override for the subtask'),
+  background: z.boolean().optional().describe('If true, return immediately without blocking on completion'),
 });
 
 type NewTaskInputT = z.infer<typeof NewTaskInput>;
@@ -30,6 +31,7 @@ export const newTaskTool: ToolDef<NewTaskInputT> = {
           input: { type: 'string', description: 'Task description for the child subtask' },
           agent: { type: 'string', description: 'Optional: dispatch to a specific agent' },
           model: { type: 'string', description: 'Optional: model override for the subtask' },
+          background: { type: 'boolean', description: 'If true, returns immediately without waiting' },
         },
         required: ['input'],
       },
@@ -50,6 +52,7 @@ export const newTaskTool: ToolDef<NewTaskInputT> = {
       return { error: 'Cannot determine project_id from current session' };
     }
 
+    const isBg = input.background === true;
     const [task] = await sql<{ id: string; state: string }[]>`
       INSERT INTO tasks (project_id, parent_task_id, input, agent, model)
       VALUES (${session.project_id}, ${currentTaskId}, ${input.input}, ${input.agent ?? null}, ${input.model ?? null})
@@ -57,9 +60,12 @@ export const newTaskTool: ToolDef<NewTaskInputT> = {
     `;
 
     return {
-      message: `Subtask created (id: ${task!.id}). It will run in isolation. Use check_task_status to monitor.`,
+      message: isBg
+        ? `Background subtask created (id: ${task!.id}). It will continue independently.`
+        : `Subtask created (id: ${task!.id}). It will run in isolation. Use check_task_status to monitor.`,
       task_id: task!.id,
       state: task!.state,
+      background: isBg,
     };
   },
 };

packages/contracts/src/agent-capabilities.ts

@@ -0,0 +1,9 @@
+import { z } from 'zod';
+
+export const AgentCapabilitiesSchema = z.object({
+  supportsStreaming: z.boolean().default(true),
+  supportsReasoningStream: z.boolean().default(false),
+  supportsBackgroundExecution: z.boolean().default(false),
+});
+
+export type AgentCapabilities = z.infer<typeof AgentCapabilitiesSchema>;

packages/contracts/src/provider-snapshot.ts

@@ -49,4 +49,6 @@ export interface ProviderSnapshotEntry {
   commands: AgentCommand[];
   error?: string;
   fetchedAt?: string;
+  supportsStreaming?: boolean;
+  supportsReasoningStream?: boolean;
 }