fix: bind stdio to its exact fresh identity

packages/core/agent-loop/README.md

@@ -14,7 +14,7 @@ The caller fiber and the AgentLoop provider are co-owners. `AgentFactory.createA
 
 Each agent and its session share one caller-chosen `SessionId`, assumed globally unique; accidental UUID collisions are outside the supported model. Two concurrent operations with the same id may both prepare, but the final `enter()` calls arbitrate publication and every loser rolls its private resources back. Each detach is bound to the exact entered object, so a stale disposer cannot remove a later same-id replacement. A detach requested during a synchronous creation notification waits for that dispatch to unwind, preserving created/disposed pairing. Teardown runs stop and drain (including outstanding idle-injection flushes) → detach agent → detach session → unwind scope; the id becomes reusable at detach even if private scope cleanup is still finishing. Ordinary non-vetoing `agent/*` notifications go through `agentEvents(ctx, agent)`, per-step assembly goes through `assembleContextFor(agent)`, and turn-end durability checkpoints go through `ctx.sessions.flush(session)`.
 
-- `ctx.agentLoop.create(id: SessionId, options?: AgentOptions, meta?: { cwd?: string }): ReactLoopAgent` — synchronous no-setup create under the exact shared agent/session id, disposed with the calling fiber. Declarative config treats `agents[].id` as a stable label and mints `${label}-session-<uuid>` before calling this boundary; `resumeSessionId` instead loads and registers the exact persisted id. This keeps fresh restarts collision-free without retaining a second live routing identity.
+- `ctx.agentLoop.create(id: SessionId, options?: AgentOptions, meta?: { cwd?: string }): ReactLoopAgent` — synchronous no-setup create under the exact shared agent/session id, disposed with the calling fiber. Declarative config treats `agents[].id` as a stable label and normally mints `${label}-session-<uuid>` before calling this boundary; an app may instead supply an exact fresh `sessionId` when another coupled component must bind to it. `resumeSessionId` loads and registers the exact persisted id and is mutually exclusive with `sessionId`. This keeps default fresh restarts collision-free without retaining a second live routing identity.
 
 `AgentLoop` also implements the `AgentFactory` seam and registers itself via `ctx.agents.setFactory(this)`, so plugins create/resume agents through `ctx.agents` (the interface):
 
@@ -33,6 +33,7 @@ The config-driven `ctx.agentLoop.create()` path keeps its agent owned by the loo
 interface Config {
   agents: Array<{
     id: string                 // required stable label; prefixes fresh combined ids
+    sessionId?: string         // optional exact identity for a fresh session
     model?: string
     resumeSessionId?: string   // load this persisted session instead of creating one
     cwd?: string               // optional workspace cwd for the fresh session
@@ -40,7 +41,7 @@ interface Config {
 }
 ```
 
-Agents listed in config are auto-created at startup. `cwd` applies only to fresh config-created sessions; `resumeSessionId` keeps the persisted session header. Config agents have no per-agent persona field: they use `dsh-system-prompt`'s deployment default, while programmatic factory callers can register an agent-scoped `deployment:persona` shadow in `setup`. The plugin registers the built-in `model`/`cwd` prompt variables on `ctx.systemPrompt`, resolved per step from `assembleContextFor(agent)` — the helper couples the typed agent with its matching scope selector. These are runtime facts of the agents THIS loop drives, unlike the `harness:identity` and default `deployment:persona` sections, which live on `dsh-system-prompt` so they survive a swapped loop plugin.
+Agents listed in config are auto-created at startup. `cwd` and optional `sessionId` apply only to fresh config-created sessions; `resumeSessionId` keeps the persisted session header. Config agents have no per-agent persona field: they use `dsh-system-prompt`'s deployment default, while programmatic factory callers can register an agent-scoped `deployment:persona` shadow in `setup`. The plugin registers the built-in `model`/`cwd` prompt variables on `ctx.systemPrompt`, resolved per step from `assembleContextFor(agent)` — the helper couples the typed agent with its matching scope selector. These are runtime facts of the agents THIS loop drives, unlike the `harness:identity` and default `deployment:persona` sections, which live on `dsh-system-prompt` so they survive a swapped loop plugin.
 
 ### Exported concrete class
 

packages/core/agent-loop/src/index.ts

@@ -326,6 +326,8 @@ export interface Config {
   agents: (AgentOptions & {
     /** Stable config label used in logs and as the fresh combined-id prefix. */
     id: string
+    /** Optional exact identity for a fresh session; absent lets the loop mint one from the label. */
+    sessionId?: SessionId
     /** Optional workspace for a fresh session. */
     cwd?: string
     /** Persisted session to resume instead of creating a fresh session. */
@@ -341,6 +343,7 @@ export class AgentLoop extends Service implements AgentFactory {
   static Config = z.object({
     agents: z.array(z.object({
       id: z.string().required(),
+      sessionId: z.string(),
       model: z.string(),
       cwd: z.string(),
       resumeSessionId: z.string(),
@@ -360,12 +363,14 @@ export class AgentLoop extends Service implements AgentFactory {
     ctx.systemPrompt.variable('model', context => context.agent?.options.model)
     ctx.systemPrompt.variable('cwd', context => context.agent?.session.header.cwd)
 
-    for (const { id, cwd, resumeSessionId, ...options } of config.agents) {
+    for (const { id, sessionId, cwd, resumeSessionId, ...options } of config.agents) {
       if (resumeSessionId === undefined || resumeSessionId === '') {
-        const sessionId = SessionId(`${id}-session-${randomUUID()}`)
-        this.create(sessionId, options, cwd === undefined ? {} : { cwd })
+        this.create(sessionId ?? SessionId(`${id}-session-${randomUUID()}`), options, cwd === undefined ? {} : { cwd })
         continue
       }
+      if (sessionId !== undefined) {
+        throw new Error(`agent "${id}": sessionId and resumeSessionId are mutually exclusive`)
+      }
       ctx.effect(() => {
         const fiber = ctx.inject(['sessionPersistence'], (childCtx: Context) => {
           void this.resumeWith(ctx, childCtx.sessionPersistence, {

packages/core/agent-loop/tests/config-session-id.spec.ts

@@ -24,7 +24,37 @@ function waitForIdle(ctx: Context, agent: ReactLoopAgent): Promise<void> {
   })
 }
 
+async function makeCoreContext(): Promise<Context> {
+  const ctx = new Context()
+  await ctx.plugin(LlmService)
+  await ctx.plugin(SessionStore)
+  await ctx.plugin(SystemPrompt)
+  await ctx.plugin(ToolRegistry)
+  await ctx.plugin(AgentRegistry)
+  return ctx
+}
+
 describe('config-driven session id', () => {
+  it('accepts one exact fresh id and rejects it alongside a resume id', async () => {
+    const exact = await makeCoreContext()
+    await exact.plugin(AgentLoop, {
+      agents: [{ id: 'main', sessionId: SessionId('stdio-exact'), model: 'mock' }],
+    })
+    expect(exact.agents.get(SessionId('stdio-exact'))?.session.id).toBe('stdio-exact')
+    await exact.fiber.dispose()
+
+    const conflicting = await makeCoreContext()
+    await expect(conflicting.plugin(AgentLoop, {
+      agents: [{
+        id: 'main',
+        sessionId: SessionId('fresh'),
+        resumeSessionId: SessionId('persisted'),
+        model: 'mock',
+      }],
+    })).rejects.toThrow('sessionId and resumeSessionId are mutually exclusive')
+    await conflicting.fiber.dispose()
+  })
+
   it('identity-nests the deferred resume fiber under its labeled owner effect', async () => {
     const ctx = new Context()
     await ctx.plugin(LlmService)