feat(x): execute sync tools concurrently within a turn batch

Sync tools in one assistant batch now run via Promise.all instead of
sequentially — a tool pending on I/O no longer blocks its siblings.
Three coordinated changes keep the event-sourced runtime sound:

- executeAllowedTools is two-phase: invocation events are appended
  serially in source order (durable before any side effect, deterministic
  log prefix), then all sync executions run concurrently, each appending
  progress/results as they land. Per-call error and cancel semantics are
  unchanged (moved to executeSyncTool).
- append() commits through an internal queue: persist → reduce → stream
  runs to completion per batch, so file order, in-memory order, and
  stream order stay identical even while executions overlap. A failed
  commit rejects only its caller; the chain survives for siblings.
- Abort-registry state is scoped per tool call (turnId:toolCallId) via a
  wrapper, fixing two latent races: createForRun destroying a running
  sibling's tracked processes, and cleanup tearing down the turn-wide
  force-kill scope when the first tool finished.

Wire ordering is untouched: model requests already reference tool
results by the assistant message's source order, pinned by a new test.
No concurrency cap and no per-tool serialization by design; tools that
share state must tolerate racing (file edits already reject stale
writes via their search/replace precondition).

Spec §4.5/§10.5 updated. New runtime tests cover overlap (deadlock
unless concurrent), progress interleaving, sibling failure isolation,
mid-batch cancellation, and crash recovery with multiple open
invocations.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/x/packages/core/docs/turn-runtime-design.md

@@ -154,9 +154,18 @@ semantics after ambiguous interruptions.
 Tool calls may complete in any order. The next model request must always contain
 tool results in the original order emitted by the model.
 
-The initial implementation may execute sync tools sequentially for simplicity.
+Sync tools in one batch execute concurrently: invocation events are appended
-Async tools naturally complete independently. No behavior may rely on physical
+serially in source order before any execution starts, then all sync executions
-completion order.
+run at once, each appending its progress and result as it lands. Result order
+in the log is therefore physical completion order and is not deterministic
+across runs; any given log still replays deterministically. Async tools
+naturally complete independently. No behavior may rely on physical completion
+order.
+
+Durable appends are serialized through a single internal queue per invocation:
+the persist → reduce → stream ritual runs to completion for one batch of
+events before the next begins, so file order, in-memory order, and stream
+order are identical by construction even while executions overlap.
 
 ## 5. Storage design
 
@@ -904,7 +913,12 @@ For one completed assistant response:
 2. Determine permission requirements.
 3. Apply automatic permission decisions when enabled.
 4. Advance each tool independently as its permission is resolved.
-5. Execute allowed sync tools using a simple sequential policy initially.
+5. Record invocations for allowed tools serially in source order (sync and
+   async alike), then execute all allowed sync tools concurrently. There is
+   no concurrency cap and no per-tool serialization; tools that share state
+   must tolerate racing (or reject stale operations, as file edits do via
+   their search/replace precondition). Secondary kill-path state (the abort
+   registry) is scoped per tool call, never per turn.
 6. Expose allowed async tool requests.
 7. Suspend when any permissions or async results remain outstanding.
 8. Once all calls have terminal results, build the next model request with

apps/x/packages/core/src/turns/bridges/real-tool-registry.test.ts

@@ -20,7 +20,9 @@ class FakeAbortRegistry implements IAbortRegistry {
         this.calls.push(`create:${runId}`);
         return new AbortController().signal;
     }
-    registerProcess(): void {}
+    registerProcess(runId: string): void {
+        this.calls.push(`register:${runId}`);
+    }
     unregisterProcess(): void {}
     abort(runId: string): void {
         this.calls.push(`abort:${runId}`);
@@ -94,8 +96,30 @@ describe("RealToolRegistry", () => {
         expect(calls[0].attachment).toEqual({ type: "builtin", name: "echo" });
         expect(calls[0].input).toEqual({ text: "hi" });
         expect(calls[0].ctx).toMatchObject({ runId: "turn-1", toolCallId: "tc-1" });
-        // Abort registry bracketed per call.
+        // Abort registry bracketed and keyed per tool call (sync tools in a
-        expect(abortRegistry.calls).toEqual(["create:turn-1", "cleanup:turn-1"]);
+        // turn execute concurrently; a shared turn key would let one call
+        // tear down its siblings' force-kill scope).
+        expect(abortRegistry.calls).toEqual([
+            "create:turn-1:tc-1",
+            "cleanup:turn-1:tc-1",
+        ]);
+    });
+
+    it("re-keys registry calls a tool makes with ctx.runId to the call scope", async () => {
+        // Builtins address the abort registry with ctx.runId (the turn id).
+        // The scoped wrapper must pin those to the per-call key, or a
+        // process registered by one tool would land in no scope at all.
+        const { registry, abortRegistry } = makeRegistry(async ({ ctx }) => {
+            ctx.abortRegistry.registerProcess(ctx.runId, {} as never);
+            return "ok";
+        });
+        const tool = (await registry.resolve(descriptor())) as SyncRuntimeTool;
+        await tool.execute({}, makeCtx());
+        expect(abortRegistry.calls).toEqual([
+            "create:turn-1:tc-1",
+            "register:turn-1:tc-1",
+            "cleanup:turn-1:tc-1",
+        ]);
     });
 
     it("normalizes undefined results to null and serializes objects", async () => {
@@ -187,9 +211,9 @@ describe("RealToolRegistry", () => {
         const tool = (await registry.resolve(descriptor())) as SyncRuntimeTool;
         await tool.execute({}, makeCtx({ signal: controller.signal }));
         expect(abortRegistry.calls).toEqual([
-            "create:turn-1",
+            "create:turn-1:tc-1",
-            "abort:turn-1",
+            "abort:turn-1:tc-1",
-            "cleanup:turn-1",
+            "cleanup:turn-1:tc-1",
         ]);
     });
 
@@ -199,7 +223,10 @@ describe("RealToolRegistry", () => {
         });
         const tool = (await registry.resolve(descriptor())) as SyncRuntimeTool;
         await expect(tool.execute({}, makeCtx())).rejects.toThrowError("tool exploded");
-        expect(abortRegistry.calls).toEqual(["create:turn-1", "cleanup:turn-1"]);
+        expect(abortRegistry.calls).toEqual([
+            "create:turn-1:tc-1",
+            "cleanup:turn-1:tc-1",
+        ]);
     });
 
     it("resolves mcp descriptors into mcp attachments (server:tool split on first colon)", async () => {

apps/x/packages/core/src/turns/bridges/real-tool-registry.ts

@@ -1,3 +1,4 @@
+import type { ChildProcess } from "child_process";
 import type { z } from "zod";
 import type { ToolAttachment } from "@x/shared/dist/agent.js";
 import type { JsonValue, ToolDescriptor } from "@x/shared/dist/turns.js";
@@ -22,6 +23,47 @@ export interface RealToolRegistryDeps {
     builtins?: typeof BuiltinTools;
 }
 
+// Sync tools within a turn execute concurrently, so abort-registry state must
+// be scoped per tool call: createForRun destroys any existing state under its
+// key, and cleanup would otherwise tear down the force-kill scope of
+// still-running siblings. Builtins address the registry with ctx.runId (the
+// turn id, which keeps its meaning elsewhere), so this wrapper pins every
+// operation to the call-scoped key regardless of the key the caller passes.
+class CallScopedAbortRegistry implements IAbortRegistry {
+    constructor(
+        private readonly inner: IAbortRegistry,
+        private readonly key: string,
+    ) {}
+
+    createForRun(): AbortSignal {
+        return this.inner.createForRun(this.key);
+    }
+
+    registerProcess(_runId: string, process: ChildProcess): void {
+        this.inner.registerProcess(this.key, process);
+    }
+
+    unregisterProcess(_runId: string, process: ChildProcess): void {
+        this.inner.unregisterProcess(this.key, process);
+    }
+
+    abort(): void {
+        this.inner.abort(this.key);
+    }
+
+    forceAbort(): void {
+        this.inner.forceAbort(this.key);
+    }
+
+    isAborted(): boolean {
+        return this.inner.isAborted(this.key);
+    }
+
+    cleanup(): void {
+        this.inner.cleanup(this.key);
+    }
+}
+
 // Bridges persisted tool descriptors to the existing dispatch: builtins via
 // the BuiltinTools catalog, MCP tools via execTool's MCP path. toolId encodes
 // the attachment: "builtin:<name>" or "mcp:<server>:<tool>". ask-human is the
@@ -89,9 +131,14 @@ export class RealToolRegistry implements IToolRegistry {
             execute: async (input, ctx: ToolExecutionContext) => {
                 // AbortSignal is the primary kill path; the abort registry is
                 // the secondary force-kill for spawned child processes,
-                // bracketed per call and keyed by turn.
+                // bracketed and keyed per tool call (sync tools in one turn
-                this.abortRegistry.createForRun(ctx.turnId);
+                // run concurrently).
-                const onAbort = () => this.abortRegistry.abort(ctx.turnId);
+                const abortRegistry: IAbortRegistry = new CallScopedAbortRegistry(
+                    this.abortRegistry,
+                    `${ctx.turnId}:${ctx.toolCallId}`,
+                );
+                abortRegistry.createForRun(ctx.turnId);
+                const onAbort = () => abortRegistry.abort(ctx.turnId);
                 ctx.signal.addEventListener("abort", onAbort, { once: true });
                 try {
                     const value = await this.execToolImpl(
@@ -101,7 +148,7 @@ export class RealToolRegistry implements IToolRegistry {
                             runId: ctx.turnId,
                             toolCallId: ctx.toolCallId,
                             signal: ctx.signal,
-                            abortRegistry: this.abortRegistry,
+                            abortRegistry,
                             publish: async (event) => {
                                 if (event.type === "tool-output-stream") {
                                     await ctx.reportProgress({
@@ -147,7 +194,7 @@ export class RealToolRegistry implements IToolRegistry {
                     };
                 } finally {
                     ctx.signal.removeEventListener("abort", onAbort);
-                    this.abortRegistry.cleanup(ctx.turnId);
+                    abortRegistry.cleanup(ctx.turnId);
                 }
             },
         };

apps/x/packages/core/src/turns/runtime.test.ts

@@ -1757,3 +1757,317 @@ describe("tool progress", () => {
         ]);
     });
 });
+
+describe("concurrent sync tool execution (10.5)", () => {
+    const slowDescriptor: z.infer<typeof ToolDescriptor> = {
+        toolId: "tool.slow",
+        name: "slow",
+        description: "Slow tool",
+        inputSchema: {},
+        execution: "sync",
+        requiresHuman: false,
+    };
+    const fastDescriptor: z.infer<typeof ToolDescriptor> = {
+        toolId: "tool.fast",
+        name: "fast",
+        description: "Fast tool",
+        inputSchema: {},
+        execution: "sync",
+        requiresHuman: false,
+    };
+    const agent: z.infer<typeof ResolvedAgent> = {
+        ...defaultAgent,
+        tools: [slowDescriptor, fastDescriptor],
+    };
+
+    it("executes a batch concurrently: invocations in source order, results in completion order", async () => {
+        // slow (first in source order) only finishes after fast has fully
+        // completed AND reported progress. Under the old sequential loop this
+        // deadlocks (fast never starts), so settling at all proves overlap.
+        const order: string[] = [];
+        let releaseSlow!: () => void;
+        const slowGate = new Promise<void>((resolve) => {
+            releaseSlow = resolve;
+        });
+        const tools: RuntimeTool[] = [
+            syncTool(slowDescriptor, async () => {
+                order.push("slow:start");
+                await slowGate;
+                order.push("slow:end");
+                return { output: "slow-done", isError: false };
+            }),
+            syncTool(fastDescriptor, async (_input, ctx) => {
+                order.push("fast:start");
+                await ctx.reportProgress({ note: "while slow is pending" });
+                order.push("fast:end");
+                releaseSlow();
+                return { output: "fast-done", isError: false };
+            }),
+        ];
+        const { runtime, repo, models } = makeRuntime({
+            agent,
+            tools,
+            models: [
+                respond(
+                    completedResp(
+                        assistantCalls(
+                            toolCallPart("S", "slow"),
+                            toolCallPart("F", "fast"),
+                        ),
+                    ),
+                ),
+                respond(completedResp(assistantText("done"))),
+            ],
+        });
+        const turnId = await newTurn(runtime);
+        const { outcome } = await advanceAndSettle(runtime, turnId);
+        expect(outcome?.status).toBe("completed");
+        expect(order).toEqual(["slow:start", "fast:start", "fast:end", "slow:end"]);
+
+        // The log stays legal under interleaving: both invocations precede
+        // any result (source order), fast's progress and result land while
+        // slow is still open, and the reducer accepts the whole history.
+        const log = await persisted(repo, turnId);
+        expect(typesOf(log)).toEqual([
+            "turn_created",
+            "model_call_requested",
+            "model_call_completed",
+            "tool_invocation_requested",
+            "tool_invocation_requested",
+            "tool_progress",
+            "tool_result",
+            "tool_result",
+            "model_call_requested",
+            "model_call_completed",
+            "turn_completed",
+        ]);
+        const invocations = log.filter(
+            (e) => e.type === "tool_invocation_requested",
+        );
+        expect(invocations.map((e) => e.toolCallId)).toEqual(["S", "F"]);
+        const results = log.filter((e) => e.type === "tool_result");
+        expect(results.map((e) => e.toolCallId)).toEqual(["F", "S"]);
+        const state = reduceTurn(log);
+        expect(state.toolCalls.map((tc) => tc.result?.result.output)).toEqual([
+            "slow-done",
+            "fast-done",
+        ]);
+
+        // Wire ordering is insulated from completion order: the follow-up
+        // request references tool results in the assistant message's source
+        // order, and the composed payload sends them in that order.
+        const followUp = log.find(
+            (e) => e.type === "model_call_requested" && e.modelCallIndex === 1,
+        );
+        expect(followUp).toMatchObject({
+            request: {
+                messages: ["assistant:0", "toolResult:S", "toolResult:F"],
+            },
+        });
+        const sent = sentMessages(models.requests[1]);
+        expect(
+            sent
+                .filter((m) => m.role === "tool")
+                .map((m) => (m as { toolCallId?: string }).toolCallId),
+        ).toEqual(["S", "F"]);
+    });
+
+    it("one tool's failure never disturbs its concurrent siblings", async () => {
+        let releaseSlow!: () => void;
+        const slowGate = new Promise<void>((resolve) => {
+            releaseSlow = resolve;
+        });
+        const tools: RuntimeTool[] = [
+            syncTool(slowDescriptor, async () => {
+                await slowGate;
+                return { output: "slow-done", isError: false };
+            }),
+            syncTool(fastDescriptor, async () => {
+                releaseSlow();
+                throw new Error("fast exploded");
+            }),
+        ];
+        const { runtime, repo } = makeRuntime({
+            agent,
+            tools,
+            models: [
+                respond(
+                    completedResp(
+                        assistantCalls(
+                            toolCallPart("S", "slow"),
+                            toolCallPart("F", "fast"),
+                        ),
+                    ),
+                ),
+                respond(completedResp(assistantText("done"))),
+            ],
+        });
+        const turnId = await newTurn(runtime);
+        const { outcome } = await advanceAndSettle(runtime, turnId);
+        expect(outcome?.status).toBe("completed");
+        const log = await persisted(repo, turnId);
+        const byId = new Map(
+            log
+                .filter((e) => e.type === "tool_result")
+                .map((e) => [e.toolCallId, e]),
+        );
+        expect(byId.get("F")).toMatchObject({
+            result: { output: "fast exploded", isError: true },
+        });
+        expect(byId.get("S")).toMatchObject({
+            result: { output: "slow-done", isError: false },
+        });
+    });
+
+    it("cancellation mid-batch settles every in-flight tool", async () => {
+        const controller = new AbortController();
+        const started: string[] = [];
+        function hangingTool(name: string) {
+            return async (
+                _input: unknown,
+                ctx: ToolExecutionContext,
+            ): Promise<{ output: string; isError: boolean }> => {
+                started.push(name);
+                if (started.length === 2) {
+                    controller.abort();
+                }
+                await new Promise<void>((resolve) => {
+                    if (ctx.signal.aborted) {
+                        resolve();
+                    } else {
+                        ctx.signal.addEventListener("abort", () => resolve(), {
+                            once: true,
+                        });
+                    }
+                });
+                throw new Error("aborted");
+            };
+        }
+        const tools: RuntimeTool[] = [
+            syncTool(slowDescriptor, hangingTool("slow")),
+            syncTool(fastDescriptor, hangingTool("fast")),
+        ];
+        const { runtime, repo } = makeRuntime({
+            agent,
+            tools,
+            models: [
+                respond(
+                    completedResp(
+                        assistantCalls(
+                            toolCallPart("S", "slow"),
+                            toolCallPart("F", "fast"),
+                        ),
+                    ),
+                ),
+            ],
+        });
+        const turnId = await newTurn(runtime);
+        const { outcome } = await advanceAndSettle(runtime, turnId, undefined, {
+            signal: controller.signal,
+        });
+        expect(outcome?.status).toBe("cancelled");
+        expect(started).toEqual(["slow", "fast"]);
+        const log = await persisted(repo, turnId);
+        const results = log.filter((e) => e.type === "tool_result");
+        expect(results).toHaveLength(2);
+        for (const result of results) {
+            expect(result).toMatchObject({
+                result: {
+                    output: "Tool execution was cancelled.",
+                    isError: true,
+                },
+            });
+        }
+        expect(typesOf(log)).toContain("turn_cancelled");
+    });
+
+    it("recovers a crash that left multiple sync invocations open", async () => {
+        const SEED_ID = "2026-07-02T10-00-00Z-0000001-000";
+        const repo = new InMemoryTurnRepo();
+        const batch = assistantCalls(
+            toolCallPart("S", "slow"),
+            toolCallPart("F", "fast"),
+        );
+        repo.seed([
+            {
+                type: "turn_created",
+                schemaVersion: 1,
+                turnId: SEED_ID,
+                ts: TS,
+                sessionId: null,
+                agent: { requested: { agentId: "copilot" }, resolved: agent },
+                context: [],
+                input: user("hello"),
+                config: {
+                    autoPermission: false,
+                    humanAvailable: true,
+                    maxModelCalls: 20,
+                },
+            },
+            {
+                type: "model_call_requested",
+                turnId: SEED_ID,
+                ts: TS,
+                modelCallIndex: 0,
+                request: { messages: ["input"], parameters: {} },
+            },
+            {
+                type: "model_call_completed",
+                turnId: SEED_ID,
+                ts: TS,
+                modelCallIndex: 0,
+                message: batch,
+                finishReason: "stop",
+                usage: {},
+            },
+            {
+                type: "tool_invocation_requested",
+                turnId: SEED_ID,
+                ts: TS,
+                toolCallId: "S",
+                toolId: "tool.slow",
+                toolName: "slow",
+                execution: "sync",
+                input: {},
+            },
+            {
+                type: "tool_invocation_requested",
+                turnId: SEED_ID,
+                ts: TS,
+                toolCallId: "F",
+                toolId: "tool.fast",
+                toolName: "fast",
+                execution: "sync",
+                input: {},
+            },
+        ]);
+        const { runtime } = makeRuntime({
+            repo,
+            agent,
+            tools: [
+                syncTool(slowDescriptor, async () => ({
+                    output: "never",
+                    isError: false,
+                })),
+                syncTool(fastDescriptor, async () => ({
+                    output: "never",
+                    isError: false,
+                })),
+            ],
+            models: [respond(completedResp(assistantText("done")))],
+        });
+        const { outcome } = await advanceAndSettle(runtime, SEED_ID);
+        expect(outcome?.status).toBe("completed");
+        const log = await persisted(repo, SEED_ID);
+        const indeterminate = log.filter(
+            (e) =>
+                e.type === "tool_result" &&
+                e.source === "runtime" &&
+                e.result.isError === true,
+        );
+        expect(indeterminate.map((e) => (e as { toolCallId: string }).toolCallId).sort()).toEqual([
+            "F",
+            "S",
+        ]);
+    });
+});

apps/x/packages/core/src/turns/runtime.ts

@@ -346,8 +346,24 @@ class TurnAdvance {
     }
 
     // Durable barrier: persist, re-reduce (the reducer doubles as a runtime
-    // assertion that the appended history is legal), then stream.
+    // assertion that the appended history is legal), then stream. Commits are
-    private async append(...batch: TEvent[]): Promise<void> {
+    // serialized through an internal queue so concurrently executing tools
+    // can never interleave the persist/reduce/stream ritual — file order,
+    // in-memory order, and stream order stay identical by construction.
+    private appendChain: Promise<void> = Promise.resolve();
+
+    private append(...batch: TEvent[]): Promise<void> {
+        const task = this.appendChain.then(() => this.commit(batch));
+        // A failed commit rejects for its caller only; the chain stays alive
+        // so other in-flight tools can still record their results.
+        this.appendChain = task.then(
+            () => undefined,
+            () => undefined,
+        );
+        return task;
+    }
+
+    private async commit(batch: TEvent[]): Promise<void> {
         await this.turnRepo.append(this.turnId, batch);
         this.events.push(...batch);
         this.state = reduceTurn(this.events);
@@ -741,8 +757,13 @@ class TurnAdvance {
         }
     }
 
-    // §10.5: execute allowed sync tools sequentially and expose allowed async
+    // §10.5: record invocations for allowed tools serially in source order,
-    // tools, in source order. Tool failures are conversational, not terminal.
+    // then execute the sync ones concurrently (async tools are exposed by
+    // their invocation; results arrive through advanceTurn). Invocations are
+    // durable before any execution starts, and commits are serialized by
+    // append's internal queue, so the log prefix is deterministic while
+    // results land in completion order. Tool failures are conversational,
+    // not terminal.
     private async executeAllowedTools(): Promise<void> {
         const executable = this.state.toolCalls.filter(
             (tc) =>
@@ -751,8 +772,11 @@ class TurnAdvance {
                 (this.checkerAllowed.has(tc.toolCallId) ||
                     tc.permission?.resolved?.decision === "allow"),
         );
+        const started: Array<{ tc: ToolCallState; tool: SyncRuntimeTool }> = [];
         for (const tc of executable) {
             if (this.signal.aborted) {
+                // Invoked-but-unexecuted calls get their cancelled results
+                // from cancel(), same as before this loop ran.
                 return;
             }
             const tool = this.toolsByName.get(tc.toolName);
@@ -771,52 +795,63 @@ class TurnAdvance {
             if (tool.descriptor.execution === "async") {
                 continue; // exposed; the result arrives through advanceTurn
             }
-            const syncTool = tool as SyncRuntimeTool;
+            started.push({ tc, tool: tool as SyncRuntimeTool });
-            try {
+        }
-                const result = await syncTool.execute(tc.input, {
+        // Each task settles its own call (result or error), so Promise.all
-                    turnId: this.turnId,
+        // never rejects and one slow tool never blocks its siblings.
-                    toolCallId: tc.toolCallId,
+        await Promise.all(
-                    signal: this.signal,
+            started.map(({ tc, tool }) => this.executeSyncTool(tc, tool)),
-                    reportProgress: async (progress) => {
+        );
-                        await this.append({
+    }
-                            type: "tool_progress",
+
-                            turnId: this.turnId,
+    private async executeSyncTool(
-                            ts: this.now(),
+        tc: ToolCallState,
-                            toolCallId: tc.toolCallId,
+        syncTool: SyncRuntimeTool,
-                            source: "sync",
+    ): Promise<void> {
-                            progress,
+        try {
-                        });
+            const result = await syncTool.execute(tc.input, {
-                    },
+                turnId: this.turnId,
-                });
+                toolCallId: tc.toolCallId,
-                await this.append({
+                signal: this.signal,
-                    type: "tool_result",
+                reportProgress: async (progress) => {
-                    turnId: this.turnId,
+                    await this.append({
-                    ts: this.now(),
+                        type: "tool_progress",
-                    toolCallId: tc.toolCallId,
+                        turnId: this.turnId,
-                    toolName: tc.toolName,
+                        ts: this.now(),
-                    source: "sync",
+                        toolCallId: tc.toolCallId,
-                    result: ToolResultData.parse(result),
+                        source: "sync",
-                });
+                        progress,
-            } catch (error) {
+                    });
-                if (this.signal.aborted) {
+                },
-                    await this.append(
+            });
-                        runtimeResultEvent(this.turnId, this.now(), tc, {
+            await this.append({
-                            output: "Tool execution was cancelled.",
+                type: "tool_result",
-                            isError: true,
+                turnId: this.turnId,
-                        }),
+                ts: this.now(),
-                    );
+                toolCallId: tc.toolCallId,
-                    return;
+                toolName: tc.toolName,
-                }
+                source: "sync",
-                await this.append({
+                result: ToolResultData.parse(result),
-                    type: "tool_result",
+            });
-                    turnId: this.turnId,
+        } catch (error) {
-                    ts: this.now(),
+            if (this.signal.aborted) {
-                    toolCallId: tc.toolCallId,
+                await this.append(
-                    toolName: tc.toolName,
+                    runtimeResultEvent(this.turnId, this.now(), tc, {
-                    source: "sync",
+                        output: "Tool execution was cancelled.",
-                    result: { output: errorMessage(error), isError: true },
+                        isError: true,
-                });
+                    }),
+                );
+                return;
             }
+            await this.append({
+                type: "tool_result",
+                turnId: this.turnId,
+                ts: this.now(),
+                toolCallId: tc.toolCallId,
+                toolName: tc.toolName,
+                source: "sync",
+                result: { output: errorMessage(error), isError: true },
+            });
         }
     }