diff --git a/apps/desktop/src/main/__tests__/workhub-runtime.test.ts b/apps/desktop/src/main/__tests__/workhub-runtime.test.ts
index b5bde91bf1..6dc28d7cf2 100644
--- a/apps/desktop/src/main/__tests__/workhub-runtime.test.ts
+++ b/apps/desktop/src/main/__tests__/workhub-runtime.test.ts
@@ -20,6 +20,7 @@
 import assert from 'node:assert/strict';
 import test from 'node:test';
 import { WORKHUB_COORDINATION_SESSION_ID } from '@maka/core/session';
+import { decodeWorkHubCoordinationActFromTurnInput, decodeWorkHubCoordinationSelectAndDelegateInput } from '@maka/runtime-host/protocol';
 import { workHubTasksSchema } from '../../shared/workhub-tool-schema.js';
 import { createWorkHubRuntime } from '../workhub-runtime.js';
 
@@ -60,6 +61,38 @@ test('task delegation binds the tool action to the Host turn and trusted creatio
   assert.deepEqual(f.changes, [[scope, 'created', 'target']]);
 });
 
+test('nested tool calls produce stable task identities accepted by both Host action paths', async () => {
+  const f = fixture();
+  const toolCallId = `${'p'.repeat(128)}:nested:00000000-0000-4000-8000-000000000001`;
+  const actionIds: string[] = [];
+  const result = { disposition: 'create_new' as const, targetSessionId: 'target', targetTurnId: 'target-turn' };
+  f.client.actWorkHubCoordinationFromTurn = async (input) => {
+    actionIds.push(decodeWorkHubCoordinationActFromTurnInput(input).actionId);
+    return result;
+  };
+  f.client.selectAndDelegateWorkHubTarget = async (input) => {
+    actionIds.push(decodeWorkHubCoordinationSelectAndDelegateInput(input).actionId);
+    return { kind: 'delegated', result };
+  };
+
+  const create = { operation: 'create_new' as const, title: 'Work', text: 'Do work' };
+  const created = await f.runtime.actTasks(scope, 'turn', toolCallId, create);
+  const retried = await f.runtime.actTasks(scope, 'turn', toolCallId, create);
+  const selected = await f.runtime.actTasks(scope, 'turn', toolCallId, {
+    operation: 'select_and_delegate', candidateSetId: `sha256:${'a'.repeat(64)}`,
+    candidateRefs: ['candidate'], text: 'Do work',
+  });
+  for (const outcome of [created, retried, selected]) {
+    assert.ok('actionId' in outcome);
+    assert.equal(outcome.actionId, actionIds[0]);
+  }
+  assert.deepEqual(actionIds, [actionIds[0], actionIds[0], actionIds[0]]);
+  const next = await f.runtime.actTasks(scope, 'turn', toolCallId.replace(/1$/, '2'), create);
+  assert.ok('actionId' in next);
+  assert.equal(next.actionId, actionIds[3]);
+  assert.notEqual(next.actionId, actionIds[0]);
+});
+
 test('linked task operations remain operations at the Host protocol boundary', async () => {
   const f = fixture();
   await f.runtime.actTasks(scope, 'turn', 'correct-action', {
diff --git a/apps/desktop/src/main/workhub-runtime.ts b/apps/desktop/src/main/workhub-runtime.ts
index 22a4c185cf..3f318ea8e0 100644
--- a/apps/desktop/src/main/workhub-runtime.ts
+++ b/apps/desktop/src/main/workhub-runtime.ts
@@ -18,6 +18,7 @@
  */
 
 import { WORKHUB_COORDINATION_SESSION_ID, type WorkHubCreateDefaults } from '@maka/core/session';
+import { clientCapabilityEntityId } from '@maka/runtime-host/client-capability-entity-id';
 import type { WorkHubCoordinationProposal, WorkspaceTarget } from '@maka/runtime-host/protocol';
 import { desktopSessionKey, type DesktopTargetScope } from '../shared/runtime-host-identity.js';
 import type { WorkHubTasksInput } from '../shared/workhub-tool-schema.js';
@@ -55,10 +56,12 @@ export function createWorkHubRuntime(deps: WorkHubRuntimeDeps) {
       const turn = await queryTurn(client, turnId);
       if (isLive(turn)) await client.stopTurn({ sessionId: turn.sessionId, turnId: turn.turnId, runId: turn.runId });
     },
-    async actTasks(scope: DesktopTargetScope, turnId: string, actionId: string, input: WorkHubTasksInput) {
+    async actTasks(scope: DesktopTargetScope, turnId: string, toolCallId: string, input: WorkHubTasksInput) {
       requireCurrent(scope);
       const client = deps.client(scope);
       if (input.operation === 'candidates') return client.listWorkHubCoordinationCandidates();
+      // WorkHub persists actions as entities; capability tool-call IDs are opaque.
+      const actionId = clientCapabilityEntityId(toolCallId);
       if (input.operation === 'select_and_delegate') {
         const outcome = await client.selectAndDelegateWorkHubTarget({ turnId, actionId,
           candidateSetId: input.candidateSetId, candidateRefs: input.candidateRefs, delegationText: input.text });
diff --git a/packages/runtime-host/src/__tests__/client-capability-protocol.test.ts b/packages/runtime-host/src/__tests__/client-capability-protocol.test.ts
index f9a5a55381..332829d622 100644
--- a/packages/runtime-host/src/__tests__/client-capability-protocol.test.ts
+++ b/packages/runtime-host/src/__tests__/client-capability-protocol.test.ts
@@ -31,6 +31,44 @@ import {
 } from '../protocol/index.js';
 
 describe('Client Capability protocol', () => {
+  test('preserves opaque tool-call IDs while retaining identity bounds', () => {
+    const frame = {
+      kind: 'client.capability.call',
+      invocationId: 'invocation',
+      registrationId: 'registration',
+      offerId: 'offer',
+      serverId: 'server',
+      toolName: 'tool',
+      arguments: {},
+      sessionId: 'session',
+      turnId: 'turn',
+    };
+    for (const toolCallId of ['call:outer:nested:inner', 'provider/call.1+part', 'x'.repeat(256)]) {
+      assert.deepEqual(decodeHostFrame({ ...frame, toolCallId }), { ...frame, toolCallId });
+    }
+    for (const toolCallId of [
+      undefined,
+      null,
+      1,
+      '',
+      'x'.repeat(257),
+      ' leading',
+      'trailing ',
+      'a\nb',
+      'a\u0000b',
+      'a\u007fb',
+    ]) {
+      assert.throws(() => decodeHostFrame({ ...frame, toolCallId }), RuntimeHostProtocolError);
+    }
+    for (const field of ['invocationId', 'registrationId', 'offerId', 'sessionId', 'turnId']) {
+      assert.throws(
+        () =>
+          decodeHostFrame({ ...frame, toolCallId: 'call:nested:inner', [field]: 'entity:invalid' }),
+        RuntimeHostProtocolError,
+      );
+    }
+  });
+
   test('decodes open-world registration and reverse-call lifecycle frames', () => {
     assert.deepEqual(
       decodeClientFrame({
diff --git a/packages/runtime-host/src/__tests__/client-capability-uds.test.ts b/packages/runtime-host/src/__tests__/client-capability-uds.test.ts
index b99987ebe1..737a9390d9 100644
--- a/packages/runtime-host/src/__tests__/client-capability-uds.test.ts
+++ b/packages/runtime-host/src/__tests__/client-capability-uds.test.ts
@@ -126,6 +126,9 @@ test('unknown Client Capability loads, invokes, and rebinds after UDS reconnect'
     await client.status();
 
     const largeValue = 'x'.repeat(100_000);
+    // Code Mode appends its nested identity to a provider-owned call ID.
+    // This crosses both the entity-ID alphabet and its 128-character limit.
+    const toolCallId = `${'p'.repeat(128)}:nested:00000000-0000-4000-8000-000000000001`;
     let providerCloseCalls = 0;
     const provider: ClientCapabilityProvider = {
       offers: () => [
@@ -161,6 +164,7 @@ test('unknown Client Capability loads, invokes, and rebinds after UDS reconnect'
         },
       ],
       call: async (frame, { accept }) => {
+        assert.equal(frame.toolCallId, toolCallId);
         if (frame.toolName === 'reject_unknown') {
           throw new Error('Provider rejected before acceptance');
         }
@@ -201,7 +205,7 @@ test('unknown Client Capability loads, invokes, and rebinds after UDS reconnect'
       sessionId: 'session-uds',
       turnId: 'turn-uds',
       cwd: root,
-      toolCallId: 'tool-call-uds',
+      toolCallId,
       abortSignal: new AbortController().signal,
       emitOutput: () => undefined,
     };
@@ -238,6 +242,7 @@ test('unknown Client Capability loads, invokes, and rebinds after UDS reconnect'
     assert.deepEqual(result, {
       content: [{ type: 'text', text: `from-uds:${largeValue}` }],
     });
+    await client.status();
     await assert.rejects(
       async () => rejectedTool.impl({}, toolContext),
       (error: unknown) =>
@@ -269,6 +274,7 @@ test('unknown Client Capability loads, invokes, and rebinds after UDS reconnect'
     await client.replaceClientCapabilities({
       offers: provider.offers,
       call: async (frame, { accept }) => {
+        assert.equal(frame.toolCallId, toolCallId);
         await accept({ kind: 'none' });
         return {
           content: [{ type: 'text', text: `reconnected:${String(frame.arguments.prefix)}` }],
diff --git a/packages/runtime-host/src/protocol/agent-graph.ts b/packages/runtime-host/src/protocol/agent-graph.ts
index 02c9842c00..706115ff76 100644
--- a/packages/runtime-host/src/protocol/agent-graph.ts
+++ b/packages/runtime-host/src/protocol/agent-graph.ts
@@ -22,6 +22,7 @@ import {
   requireEncodedByteLimit,
   requireEntityId,
   requireExactRecord,
+  requireOpaqueIdentity,
   requireShapedRecord,
   requireUtf8String,
 } from './codec.js';
@@ -1348,19 +1349,6 @@ function requireBoolean(value: unknown, label: string): boolean {
   return value;
 }
 
-function requireOpaqueIdentity(value: unknown, label: string): string {
-  if (
-    typeof value !== 'string' ||
-    value.length === 0 ||
-    value.length > 256 ||
-    value.trim() !== value ||
-    /[\u0000-\u001f\u007f]/.test(value)
-  ) {
-    throw invalidProtocolFrame(`Invalid ${label}`);
-  }
-  return value;
-}
-
 function requireFingerprint(value: unknown, label: string): `sha256:${string}` {
   if (typeof value !== 'string' || !/^sha256:[a-f0-9]{64}$/.test(value)) {
     throw invalidProtocolFrame(`Invalid ${label}`);
diff --git a/packages/runtime-host/src/protocol/client-capability.ts b/packages/runtime-host/src/protocol/client-capability.ts
index f50025487c..643c1b4e9c 100644
--- a/packages/runtime-host/src/protocol/client-capability.ts
+++ b/packages/runtime-host/src/protocol/client-capability.ts
@@ -29,6 +29,7 @@ import {
   requireCount,
   requireEntityId,
   requireExactRecord,
+  requireOpaqueIdentity,
   requireRecord,
   requireString,
 } from './codec.js';
@@ -580,7 +581,7 @@ export function decodeClientCapabilityHostFrame(value: unknown): ClientCapabilit
         arguments: argumentsValue,
         sessionId: requireEntityId(frame.sessionId, 'sessionId'),
         turnId: requireEntityId(frame.turnId, 'turnId'),
-        toolCallId: requireEntityId(frame.toolCallId, 'toolCallId'),
+        toolCallId: requireOpaqueIdentity(frame.toolCallId, 'toolCallId'),
         ...(frame.cwd === undefined ? {} : { cwd: requireString(frame.cwd, 'cwd', 4_096) }),
       };
     }
diff --git a/packages/runtime-host/src/protocol/codec.ts b/packages/runtime-host/src/protocol/codec.ts
index b1288ea309..62d00ec342 100644
--- a/packages/runtime-host/src/protocol/codec.ts
+++ b/packages/runtime-host/src/protocol/codec.ts
@@ -84,6 +84,20 @@ export function requireEntityId(value: unknown, label: string): string {
   return id;
 }
 
+/** Correlation identities are preserved verbatim, not interpreted as entity paths. */
+export function requireOpaqueIdentity(value: unknown, label: string): string {
+  if (
+    typeof value !== 'string' ||
+    value.length === 0 ||
+    value.length > 256 ||
+    value.trim() !== value ||
+    /[\u0000-\u001f\u007f]/.test(value)
+  ) {
+    throw invalidProtocolFrame(`Invalid ${label}`);
+  }
+  return value;
+}
+
 export function requireCount(value: unknown, label: string): number {
   if (!Number.isSafeInteger(value) || (value as number) < 0) {
     throw invalidProtocolFrame(`Invalid ${label}`);
diff --git a/packages/runtime-host/src/protocol/index.ts b/packages/runtime-host/src/protocol/index.ts
index 10235f989e..e3a32337e2 100644
--- a/packages/runtime-host/src/protocol/index.ts
+++ b/packages/runtime-host/src/protocol/index.ts
@@ -101,7 +101,7 @@ export const RUNTIME_HOST_REGISTRATION_SCHEMA_VERSION = 1 as const;
 export const RUNTIME_HOST_PROTOCOL_VERSION = 0 as const;
 // Increment when the same protocol version no longer guarantees safe Client-Host
 // interoperability. Mismatches are rejected before domain commands are admitted.
-export const RUNTIME_HOST_COMPATIBILITY_EPOCH = 155 as const;
+export const RUNTIME_HOST_COMPATIBILITY_EPOCH = 156 as const;
 // 154: External Session import results distinguish committed Sessions from typed source limits.
 // 153: Sessions may select plugin executors and Plugin Platform queries expose them.
 // 152: Assistant completions and transcript rows preserve interrupted responses.