fix(web): compose a forked session, and give the shell realm its provider

Two consequences of moving the agent plane behind presets, both invisible
until the host plane stopped carrying model-facing rows.

`sessions.fork` built its child with a bare `installTarget` and a `meta`
without `agentPreset`. That was harmless while every tool sat in the host
plane — the child inherited them for free. It now comes up with an EMPTY
tool set. The child composes the parent's preset instead, for the same
reason a resumed session keeps its own: the seeded history was produced
under those tools.

`bashEnv` lives in its own `dsh-bash-env` row rather than inside
`tool-bash`, so a preset that isolates the realm must compose the provider
beside its consumer; the host row is disabled here like every other
model-facing one. Nothing outside the agent plane injects `bashEnv`, so it
stays per-session.

apps/cli/config/agent-presets/core-web/agent.cordis.yml

@@ -22,6 +22,11 @@
   isolate:
     bashEnv: true
   config:
+    # The registry and its consumer share the realm: a consumer left outside
+    # would resolve the host's `bashEnv`, which this plane no longer provides.
+    - id: bash-env
+      name: '@deepseek-ai/dsh-bash-env'
+
     - id: tool-bash
       name: '@deepseek-ai/dsh-tool-bash'
 

apps/cli/config/agent-presets/standard/agent.cordis.yml

@@ -40,6 +40,11 @@
   isolate:
     bashEnv: true
   config:
+    # The registry and its consumer share the realm: a consumer left outside
+    # would resolve the host's `bashEnv`, which this plane no longer provides.
+    - id: bash-env
+      name: '@deepseek-ai/dsh-bash-env'
+
     - id: tool-bash
       name: '@deepseek-ai/dsh-tool-bash'
 

apps/cli/tests/web-agent-presets.spec.ts

@@ -154,6 +154,36 @@ describe('the shipped Web composition', () => {
   })
 })
 
+describe('a forked session', () => {
+  it('inherits the composition its seeded history was produced under', async () => {
+    const parent = await ctx.agents.create({
+      sessionId: SessionId('preset-fork-parent'),
+      meta: { agentPreset: 'core-web' },
+      setup: agentCtx => ctx.agentPresets.mount(agentCtx, 'core-web').then(() => undefined),
+    })
+    const inherited = parent.agent.session.header.agentPreset
+    const child = await ctx.agents.create({
+      sessionId: SessionId('preset-fork-child'),
+      meta: {
+        parentSession: SessionId('preset-fork-parent'),
+        seedLength: 0,
+        ...inherited === undefined ? {} : { agentPreset: inherited },
+      },
+      setup: agentCtx => ctx.agentPresets.mount(agentCtx, inherited).then(() => undefined),
+    })
+    try {
+      // Composing nothing would leave the child empty: this layer moved every
+      // model-facing row out of the host plane, so there is nothing to inherit
+      // for free any more.
+      expect(toolNames(ctx, child.agent)).toEqual(toolNames(ctx, parent.agent))
+      expect(toolNames(ctx, child.agent).length).toBeGreaterThan(0)
+    } finally {
+      await child.dispose()
+      await parent.dispose()
+    }
+  })
+})
+
 describe('a session keeps the preset it was created with', () => {
   it('refuses to adopt a live session under a different preset', async () => {
     const handle = await ctx.agents.create({

packages/bundle/web-app/cordis.patch.yml

@@ -201,6 +201,9 @@
 # absent from a surface overlay would silently reappear the day someone reorders
 # the composition.
 
+- id: bash-env
+  disabled: true
+
 - id: tool-bash
   disabled: true
 

packages/host/apiproxy/src/api-proxy.ts

@@ -1898,6 +1898,12 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
           })
         }
         const childId = `session-${randomUUID()}` as SessionId
+        // The child inherits the parent's composition for the same reason a
+        // resumed session keeps its own: the seeded history was produced under
+        // those tools, and composing anything else would strand the tool calls
+        // it already carries. Now that no model-facing row sits in the host
+        // plane, composing nothing would leave the child with no tools at all.
+        const forkComposition = await composeAgent(source.header.agentPreset)
         try {
           await ctx.agents.create({
             sessionId: childId,
@@ -1906,9 +1912,12 @@ export function createApiProxy(ctx: Context, defaults: ApiProxyDefaults): ApiPro
               ...source.header.cwd === undefined ? {} : { cwd: source.header.cwd },
               parentSession: source.id,
               seedLength: cut,
+              ...forkComposition.agentPreset === undefined
+                ? {}
+                : { agentPreset: forkComposition.agentPreset },
             },
             agentOptions,
-            setup: installTarget,
+            setup: forkComposition.setup,
           })
         } catch (error: unknown) {
           return err(request, {