diff --git a/.changeset/reject-approval-resume-of-running-turn.md b/.changeset/reject-approval-resume-of-running-turn.md
new file mode 100644
index 000000000..098edea75
--- /dev/null
+++ b/.changeset/reject-approval-resume-of-running-turn.md
@@ -0,0 +1,6 @@
+---
+"@truefoundry/trueforge": patch
+"@truefoundry/trueforge-core": patch
+---
+
+Reject approval and tool-response resumes that target a still-running turn before the cancelled-for-next-turn freeze, so an invalid resume such as a duplicate approval no longer cancels the turn it races.
diff --git a/packages/trueforge-core/src/agent-session/SessionHandle.ts b/packages/trueforge-core/src/agent-session/SessionHandle.ts
index 23acbc0e7..5c0ea56a3 100644
--- a/packages/trueforge-core/src/agent-session/SessionHandle.ts
+++ b/packages/trueforge-core/src/agent-session/SessionHandle.ts
@@ -1,6 +1,7 @@
 /**
  * Bound session handle: starts turns via {@link SessionHandle.createTurn}.
  */
+import { InvalidAgentSendInputError } from '../core/errors';
 import { newEventId } from '../core/events/schema';
 import type { AgentDefinition } from '../core/runtime/AgentDefinition';
 import { AgentThread } from '../core/runtime/AgentThread';
@@ -185,6 +186,29 @@ export class SessionHandle<
     update_session_title_if_not_exist?: string | undefined;
   }): Promise<TurnHandle<TTurnCustom>> {
     const previousTurnId = resolvePreviousTurnId(input.previous_turn_id, this.session.last_turn_id);
+
+    // Approval and tool-response items answer the required actions of a
+    // completed turn; a running previous turn cannot have any. Check that
+    // read-only BEFORE the freeze below, so an invalid resume (e.g. a
+    // duplicate approval racing the turn it already resumed) is rejected
+    // without cancelling a turn that is still executing (#508).
+    const items = input.input ?? [];
+    if (
+      previousTurnId !== null &&
+      items.length > 0 &&
+      items.every(msg => isApprovalDecisionMessage(msg) || isClientSideToolResponseMessage(msg))
+    ) {
+      const prior = await this.store.getTurn({
+        session_id: this.session.session_id,
+        turn_id: previousTurnId,
+      });
+      if (prior?.state.status === 'running') {
+        throw new InvalidAgentSendInputError(
+          `cannot resume previous turn '${previousTurnId}' while it is still running`,
+        );
+      }
+    }
+
     const previous = previousTurnId
       ? await this.freezeTurn({
           turn_id: previousTurnId,
diff --git a/packages/trueforge-core/tests/agent-session/sessions.test.ts b/packages/trueforge-core/tests/agent-session/sessions.test.ts
index 3c37ec1fc..857169939 100644
--- a/packages/trueforge-core/tests/agent-session/sessions.test.ts
+++ b/packages/trueforge-core/tests/agent-session/sessions.test.ts
@@ -1,9 +1,11 @@
+import { MAIN_THREAD_ID } from '../../src/agent-session/models/TurnRecord';
 import { EventType } from '../../src/agent-session/schemas/events';
 import { CancellationReason } from '../../src/agent-session/schemas/turn';
 import { Sessions } from '../../src/agent-session/Sessions';
 import { InMemorySessionStore } from '../../src/agent-session/store/InMemorySessionStore';
 import { TurnNotFoundError } from '../../src/agent-session/store/SessionStoreErrors';
 import { TurnHandle } from '../../src/agent-session/TurnHandle';
+import { InvalidAgentSendInputError } from '../../src/core/errors';
 import { makeAgentSpec, makeTestResolver, mintTestTurnId } from './testHelpers';
 
 describe('Sessions / SessionHandle / TurnHandle (storage + createTurn)', () => {
@@ -128,6 +130,59 @@ describe('Sessions / SessionHandle / TurnHandle (storage + createTurn)', () => {
     ).rejects.toBeInstanceOf(TurnNotFoundError);
   });
 
+  it('rejects an approval-only resume of a running turn without cancelling it', async () => {
+    const store = new InMemorySessionStore();
+    const sessions = new Sessions({ sessionStore: store });
+    const session = await sessions.create({
+      tenant_id: tenant,
+      session_id: 's1',
+      created_by: 'user-1',
+      agent: { type: 'inline', spec: makeAgentSpec() },
+    });
+    const running = await session.createTurn({
+      turn_id: mintTestTurnId(),
+      input: [{ type: EventType.USER_MESSAGE, content: 'hello' }],
+      previous_turn_id: 'none',
+      signal: new AbortController().signal,
+      resolver: makeTestResolver(),
+    });
+
+    // A duplicate or stray approval must not kill the turn it races (#508).
+    await expect(
+      session.createTurn({
+        turn_id: mintTestTurnId(),
+        input: [
+          {
+            type: EventType.USER_TOOL_APPROVAL,
+            thread_id: MAIN_THREAD_ID,
+            tool_call_id: 'call-1',
+            approval: { status: 'allow' },
+          },
+        ],
+        previous_turn_id: 'auto',
+        signal: new AbortController().signal,
+        resolver: makeTestResolver(),
+      }),
+    ).rejects.toBeInstanceOf(InvalidAgentSendInputError);
+    const untouched = await store.getTurn({ session_id: 's1', turn_id: running.id });
+    expect(untouched?.state.status).toBe('running');
+
+    // A user message still barges in and cancels the running turn.
+    const barged = await session.createTurn({
+      turn_id: mintTestTurnId(),
+      input: [{ type: EventType.USER_MESSAGE, content: 'interrupt' }],
+      previous_turn_id: 'auto',
+      signal: new AbortController().signal,
+      resolver: makeTestResolver(),
+    });
+    expect(barged.state.status).toBe('running');
+    const cancelled = await store.getTurn({ session_id: 's1', turn_id: running.id });
+    expect(cancelled?.state).toMatchObject({
+      status: 'cancelled',
+      reason: CancellationReason.CancelledForNextTurn,
+    });
+  });
+
   it('custom value vs merge-fn', async () => {
     const store = new InMemorySessionStore<{ tag: string }, { n: number }>();
     const sessions = new Sessions({ sessionStore: store });