diff --git a/src/agents/embedded-agent-runner/tool-result-context-guard.test.ts b/src/agents/embedded-agent-runner/tool-result-context-guard.test.ts
index 212e44946eba..557cd882e2c6 100644
--- a/src/agents/embedded-agent-runner/tool-result-context-guard.test.ts
+++ b/src/agents/embedded-agent-runner/tool-result-context-guard.test.ts
@@ -682,6 +682,116 @@ describe("installContextEngineLoopHook", () => {
     expect(engine.assemble).not.toHaveBeenCalled();
   });
 
+  it.each([
+    "before context processing",
+    "during upstream transform",
+    "during afterTurn resolve",
+    "during afterTurn reject",
+    "during ingestBatch resolve",
+    "during ingestBatch reject",
+    "during individual ingest resolve",
+    "during individual ingest reject",
+    "during assemble resolve",
+    "during assemble reject",
+    "when reusing cached assembly",
+    "inside the composed pressure guard",
+  ] as const)("stops before another provider request when cancelled %s", async (stage) => {
+    const controller = new AbortController();
+    const cancellation = new Error(`operator cancelled ${stage}`);
+    const cancel = () => {
+      controller.abort(cancellation);
+      if (stage.endsWith("reject")) {
+        throw new Error("context engine stopped before completing");
+      }
+    };
+    const upstream =
+      stage === "during upstream transform"
+        ? vi.fn(async (messages: AgentMessage[]) => {
+            cancel();
+            return messages;
+          })
+        : undefined;
+    const agent = makeGuardableAgent(upstream);
+    const usesBatchIngest = stage.startsWith("during ingestBatch");
+    const usesIndividualIngest = stage.startsWith("during individual ingest");
+    const engine = makeMockEngine({
+      ...(usesBatchIngest || usesIndividualIngest ? { omitAfterTurn: true } : {}),
+      ...(usesIndividualIngest ? { omitIngestBatch: true } : {}),
+      ...(stage.startsWith("during afterTurn") || stage === "inside the composed pressure guard"
+        ? { afterTurn: async () => cancel() }
+        : {}),
+      ...(usesBatchIngest
+        ? {
+            ingestBatch: async ({ messages }) => {
+              cancel();
+              return { ingestedCount: messages.length };
+            },
+          }
+        : {}),
+      ...(usesIndividualIngest
+        ? {
+            ingest: async () => {
+              cancel();
+              return { ingested: true };
+            },
+          }
+        : {}),
+      ...(stage.startsWith("during assemble")
+        ? {
+            assemble: async ({ messages }) => {
+              cancel();
+              return { messages, estimatedTokens: 0 };
+            },
+          }
+        : {}),
+    });
+    if (stage === "inside the composed pressure guard") {
+      installOwnsCompactionHookWithGuard(agent, engine, { prePromptCount: 1 });
+    } else {
+      installHook(agent, engine, 1);
+    }
+    const messages = [
+      makeUser("first"),
+      makeToolResult("call_1", "result"),
+      ...(usesIndividualIngest ? [makeToolResult("call_2", "later result")] : []),
+    ];
+    if (stage === "when reusing cached assembly") {
+      await callTransform(agent, messages);
+    }
+    if (stage === "before context processing" || stage === "when reusing cached assembly") {
+      controller.abort(cancellation);
+    }
+
+    const requestProvider = vi.fn();
+    const transform = expectDefined(agent.transformContext, "installed transform test invariant");
+    await expect(
+      Promise.resolve(transform(messages, controller.signal)).then(requestProvider),
+    ).rejects.toBe(cancellation);
+
+    expect(requestProvider).not.toHaveBeenCalled();
+    if (stage === "before context processing" || stage === "during upstream transform") {
+      expect(engine.afterTurn).not.toHaveBeenCalled();
+      expect(engine.assemble).not.toHaveBeenCalled();
+    } else if (usesIndividualIngest) {
+      expect(engine.ingest).toHaveBeenCalledTimes(1);
+      expect(engine.assemble).not.toHaveBeenCalled();
+    } else if (!stage.startsWith("during assemble") && stage !== "when reusing cached assembly") {
+      expect(engine.assemble).not.toHaveBeenCalled();
+    }
+  });
+
+  it("keeps the upstream context-transform contract when no abort signal was supplied", async () => {
+    const agent = makeGuardableAgent();
+    const engine = makeMockEngine();
+    installHook(agent, engine, 1);
+    const messages = [makeUser("first"), makeToolResult("call_1", "result")];
+    const transform = expectDefined(agent.transformContext, "installed transform test invariant");
+
+    await expect(Reflect.apply(transform, agent, [messages, undefined])).resolves.toEqual(messages);
+    expect(engine.afterTurn).toHaveBeenCalledOnce();
+    expect(engine.assemble).toHaveBeenCalledOnce();
+  });
+
   it("keeps the pressure guard active around ownsCompaction loop assembly", async () => {
     const agent = makeGuardableAgent();
     const engine = makeMockEngine();
diff --git a/src/agents/embedded-agent-runner/tool-result-context-guard.ts b/src/agents/embedded-agent-runner/tool-result-context-guard.ts
index a5b653fcd423..fd16c3e8448b 100644
--- a/src/agents/embedded-agent-runner/tool-result-context-guard.ts
+++ b/src/agents/embedded-agent-runner/tool-result-context-guard.ts
@@ -337,9 +337,11 @@ export function installContextEngineLoopHook(params: {
   const transcriptProjectionCache = new WeakMap<AgentMessage, AgentMessage>();
 
   mutableAgent.transformContext = (async (messages: AgentMessage[], signal: AbortSignal) => {
+    signal?.throwIfAborted();
     const transformed = originalTransformContext
       ? await originalTransformContext.call(mutableAgent, messages, signal)
       : messages;
+    signal?.throwIfAborted();
     const sourceMessages = Array.isArray(transformed) ? transformed : messages;
     const transcriptMessages = projectTranscriptPromptMessages(
       sourceMessages,
@@ -413,10 +415,12 @@ export function installContextEngineLoopHook(params: {
                 message,
                 isHeartbeat: params.isHeartbeat,
               });
+              signal?.throwIfAborted();
             }
           }
         }
       }
+      signal?.throwIfAborted();
       lastSeenLength = transcriptMessages.length;
       params.onAfterTurnCheckpoint?.(lastSeenLength);
       lastSourceMessages = transcriptMessages;
@@ -428,6 +432,7 @@ export function installContextEngineLoopHook(params: {
         model: modelId,
         runtimeSettings: params.runtimeSettings,
       });
+      signal?.throwIfAborted();
       if (assembled && Array.isArray(assembled.messages)) {
         const repairedMessages =
           params.repairAssembledMessages?.(assembled.messages) ?? assembled.messages;
@@ -438,6 +443,7 @@ export function installContextEngineLoopHook(params: {
       }
       lastAssembledView = null;
     } catch {
+      signal?.throwIfAborted();
       // Best-effort: any engine failure falls through to the raw source
       // messages so the tool loop still makes forward progress.
       lastSeenLength = prePromptMessageCount;