openai · seratch · Nov 21, 2025 · Nov 21, 2025
diff --git a/.changeset/bold-dancers-see.md b/.changeset/bold-dancers-see.md
@@ -0,0 +1,5 @@
+---
+'@openai/agents-core': patch
+---
+
+feat: #679 Add runInParallel option to input guardrail initialization
diff --git a/packages/agents-core/src/agent.ts b/packages/agents-core/src/agent.ts
@@ -235,8 +235,9 @@ export interface AgentConfiguration<
   mcpServers: MCPServer[];
 
   /**
-   * A list of checks that run in parallel to the agent's execution, before generating a response.
-   * Runs only if the agent is the first agent in the chain.
+   * A list of checks that run in parallel to the agent by default; set `runInParallel` to false to
+   * block LLM/tool calls until the guardrail completes. Runs only if the agent is the first agent
+   * in the chain.
    */
   inputGuardrails: InputGuardrail[];
 

diff --git a/packages/agents-core/src/guardrail.ts b/packages/agents-core/src/guardrail.ts
@@ -65,6 +65,12 @@ export interface InputGuardrail {
    * The function that performs the guardrail check
    */
   execute: InputGuardrailFunction;
+
+  /**
+   * Whether the guardrail should execute alongside the agent (true, default) or block the
+   * agent until it completes (false).
+   */
+  runInParallel?: boolean;
 }
 
 /**
@@ -105,6 +111,7 @@ export interface InputGuardrailMetadata {
  */
 export interface InputGuardrailDefinition extends InputGuardrailMetadata {
   guardrailFunction: InputGuardrailFunction;
+  runInParallel: boolean;
   run(args: InputGuardrailFunctionArgs): Promise<InputGuardrailResult>;
 }
 
@@ -114,6 +121,7 @@ export interface InputGuardrailDefinition extends InputGuardrailMetadata {
 export interface DefineInputGuardrailArgs {
   name: string;
   execute: InputGuardrailFunction;
+  runInParallel?: boolean;
 }
 
 /**
@@ -122,10 +130,12 @@ export interface DefineInputGuardrailArgs {
 export function defineInputGuardrail({
   name,
   execute,
+  runInParallel = true,
 }: DefineInputGuardrailArgs): InputGuardrailDefinition {
   return {
     type: 'input',
     name,
+    runInParallel,
     guardrailFunction: execute,
     async run(args: InputGuardrailFunctionArgs): Promise<InputGuardrailResult> {
       return {

diff --git a/packages/agents-core/src/run.ts b/packages/agents-core/src/run.ts
@@ -4,6 +4,7 @@ import {
   defineOutputGuardrail,
   InputGuardrail,
   InputGuardrailDefinition,
+  InputGuardrailResult,
   OutputGuardrail,
   OutputGuardrailDefinition,
   OutputGuardrailFunctionArgs,
@@ -603,6 +604,34 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
     AgentOutputType<unknown>
   >[];
 
+  #getInputGuardrailDefinitions<
+    TContext,
+    TAgent extends Agent<TContext, AgentOutputType>,
+  >(state: RunState<TContext, TAgent>): InputGuardrailDefinition[] {
+    return this.inputGuardrailDefs.concat(
+      state._currentAgent.inputGuardrails.map(defineInputGuardrail),
+    );
+  }
+
+  #splitInputGuardrails<
+    TContext,
+    TAgent extends Agent<TContext, AgentOutputType>,
+  >(state: RunState<TContext, TAgent>) {
+    const guardrails = this.#getInputGuardrailDefinitions(state);
+    const blocking: InputGuardrailDefinition[] = [];
+    const parallel: InputGuardrailDefinition[] = [];
+
+    for (const guardrail of guardrails) {
+      if (guardrail.runInParallel === false) {
+        blocking.push(guardrail);
+      } else {
+        parallel.push(guardrail);
+      }
+    }
+
+    return { blocking, parallel };
+  }
+
   /**
    * @internal
    * Resolves the effective model once so both run loops obey the same precedence rules.
@@ -738,8 +767,21 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
               `Running agent ${state._currentAgent.name} (turn ${state._currentTurn})`,
             );
 
+            let parallelGuardrailPromise:
+              | Promise<InputGuardrailResult[]>
+              | undefined;
             if (state._currentTurn === 1) {
-              await this.#runInputGuardrails(state);
+              const guardrails = this.#splitInputGuardrails(state);
+              if (guardrails.blocking.length > 0) {
+                await this.#runInputGuardrails(state, guardrails.blocking);
+              }
+              if (guardrails.parallel.length > 0) {
+                parallelGuardrailPromise = this.#runInputGuardrails(
+                  state,
+                  guardrails.parallel,
+                );
+                parallelGuardrailPromise.catch(() => {});
+              }
             }
 
             const turnInput = serverConversationTracker
@@ -829,6 +871,10 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
               state._currentTurnPersistedItemCount = 0;
             }
             state._currentStep = turnResult.nextStep;
+
+            if (parallelGuardrailPromise) {
+              await parallelGuardrailPromise;
+            }
           }
 
           if (
@@ -1007,8 +1053,25 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
             `Running agent ${currentAgent.name} (turn ${result.state._currentTurn})`,
           );
 
+          let guardrailError: unknown;
+          let parallelGuardrailPromise:
+            | Promise<InputGuardrailResult[]>
+            | undefined;
           if (result.state._currentTurn === 1) {
-            await this.#runInputGuardrails(result.state);
+            const guardrails = this.#splitInputGuardrails(result.state);
+            if (guardrails.blocking.length > 0) {
+              await this.#runInputGuardrails(result.state, guardrails.blocking);
+            }
+            if (guardrails.parallel.length > 0) {
+              const promise = this.#runInputGuardrails(
+                result.state,
+                guardrails.parallel,
+              );
+              parallelGuardrailPromise = promise.catch((err) => {
+                guardrailError = err;
+                return [];
+              });
+            }
           }
 
           const turnInput = serverConversationTracker
@@ -1038,6 +1101,10 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
             sessionInputUpdate,
           );
 
+          if (guardrailError) {
+            throw guardrailError;
+          }
+
           handedInputToModel = true;
           await persistStreamInputIfNeeded();
 
@@ -1064,6 +1131,9 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
             ),
             signal: options.signal,
           })) {
+            if (guardrailError) {
+              throw guardrailError;
+            }
             if (event.type === 'response_done') {
               const parsed = StreamEventResponseCompleted.parse(event);
               finalResponse = {
@@ -1080,6 +1150,13 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
             result._addItem(new RunRawModelStreamEvent(event));
           }
 
+          if (parallelGuardrailPromise) {
+            await parallelGuardrailPromise;
+            if (guardrailError) {
+              throw guardrailError;
+            }
+          }
+
           result.state._noActiveAgentRun = false;
 
           if (!finalResponse) {
@@ -1276,10 +1353,12 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
   async #runInputGuardrails<
     TContext,
     TAgent extends Agent<TContext, AgentOutputType>,
-  >(state: RunState<TContext, TAgent>) {
-    const guardrails = this.inputGuardrailDefs.concat(
-      state._currentAgent.inputGuardrails.map(defineInputGuardrail),
-    );
+  >(
+    state: RunState<TContext, TAgent>,
+    guardrailsOverride?: InputGuardrailDefinition[],
+  ): Promise<InputGuardrailResult[]> {
+    const guardrails =
+      guardrailsOverride ?? this.#getInputGuardrailDefinitions(state);
     if (guardrails.length > 0) {
       const guardrailArgs = {
         agent: state._currentAgent,
@@ -1300,6 +1379,7 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
             );
           }),
         );
+        state._inputGuardrailResults.push(...results);
         for (const result of results) {
           if (result.output.tripwireTriggered) {
             if (state._currentAgentSpan) {
@@ -1315,6 +1395,7 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
             );
           }
         }
+        return results;
       } catch (e) {
         if (e instanceof InputGuardrailTripwireTriggered) {
           throw e;
@@ -1328,6 +1409,7 @@ export class Runner extends RunHooks<any, AgentOutputType<unknown>> {
         );
       }
     }
+    return [];
   }
 
   async #runOutputGuardrails<

diff --git a/packages/agents-core/test/guardrail.test.ts b/packages/agents-core/test/guardrail.test.ts
@@ -44,6 +44,31 @@ describe('guardrail helpers', () => {
     expect(agent.outputGuardrails[0].name).toEqual('og');
   });
 
+  it('defaults input guardrails to run in parallel', () => {
+    const guardrail = defineInputGuardrail({
+      name: 'ig',
+      execute: async (_args) => ({
+        outputInfo: { ok: true },
+        tripwireTriggered: false,
+      }),
+    });
+
+    expect(guardrail.runInParallel).toBe(true);
+  });
+
+  it('uses configured runInParallel value for input guardrails', () => {
+    const guardrail = defineInputGuardrail({
+      name: 'blocking',
+      execute: async (_args) => ({
+        outputInfo: { ok: true },
+        tripwireTriggered: false,
+      }),
+      runInParallel: false,
+    });
+
+    expect(guardrail.runInParallel).toBe(false);
+  });
+
   it('executes input guardrail and returns expected result', async () => {
     const guardrailFn = vi.fn(async (_args: InputGuardrailFunctionArgs) => ({
       outputInfo: { ok: true },

diff --git a/packages/agents-core/test/run.stream.test.ts b/packages/agents-core/test/run.stream.test.ts
@@ -944,6 +944,7 @@ describe('Runner.run (streaming)', () => {
 
     const guardrail = {
       name: 'block',
+      runInParallel: false,
       execute: vi.fn().mockResolvedValue({
         tripwireTriggered: true,
         outputInfo: { reason: 'blocked' },
@@ -973,6 +974,66 @@ describe('Runner.run (streaming)', () => {
 
     expect(saveInputSpy).not.toHaveBeenCalled();
   });
+
+  it('runs blocking input guardrails before streaming starts', async () => {
+    let guardrailFinished = false;
+
+    const guardrail = {
+      name: 'blocking',
+      runInParallel: false,
+      execute: vi.fn(async () => {
+        await Promise.resolve();
+        guardrailFinished = true;
+        return {
+          tripwireTriggered: false,
+          outputInfo: { ok: true },
+        };
+      }),
+    };
+
+    class ExpectGuardrailBeforeStreamModel implements Model {
+      getResponse(_request: ModelRequest): Promise<ModelResponse> {
+        throw new Error('Unexpected call to getResponse');
+      }
+
+      async *getStreamedResponse(
+        _request: ModelRequest,
+      ): AsyncIterable<StreamEvent> {
+        expect(guardrailFinished).toBe(true);
+        yield {
+          type: 'response_done',
+          response: {
+            id: 'stream1',
+            usage: {
+              requests: 1,
+              inputTokens: 0,
+              outputTokens: 0,
+              totalTokens: 0,
+            },
+            output: [fakeModelMessage('ok')],
+          },
+        } satisfies StreamEvent;
+      }
+    }
+
+    const agent = new Agent({
+      name: 'BlockingStreamAgent',
+      model: new ExpectGuardrailBeforeStreamModel(),
+      inputGuardrails: [guardrail],
+    });
+
+    const runner = new Runner();
+    const result = await runner.run(agent, 'hi', { stream: true });
+
+    for await (const _ of result.toStream()) {
+      // consume
+    }
+    await result.completed;
+
+    expect(result.finalOutput).toBe('ok');
+    expect(result.inputGuardrailResults).toHaveLength(1);
+    expect(guardrail.execute).toHaveBeenCalledTimes(1);
+  });
 });
 
 class ImmediateStreamingModel implements Model {

diff --git a/packages/agents-core/test/run.test.ts b/packages/agents-core/test/run.test.ts
@@ -321,6 +321,49 @@ describe('Runner.run', () => {
       expect(guardrailFn).toHaveBeenCalledTimes(1);
     });
 
+    it('waits for blocking input guardrails before calling the model', async () => {
+      let guardrailCompleted = false;
+      const blockingGuardrail = {
+        name: 'blocking-ig',
+        runInParallel: false,
+        execute: vi.fn(async () => {
+          await Promise.resolve();
+          guardrailCompleted = true;
+          return { tripwireTriggered: false, outputInfo: {} };
+        }),
+      };
+
+      class ExpectGuardrailFirstModel implements Model {
+        calls = 0;
+
+        async getResponse(_request: ModelRequest): Promise<ModelResponse> {
+          this.calls++;
+          expect(guardrailCompleted).toBe(true);
+          return {
+            output: [fakeModelMessage('done')],
+            usage: new Usage(),
+          };
+        }
+
+        /* eslint-disable require-yield */
+        async *getStreamedResponse(_request: ModelRequest) {
+          throw new Error('not implemented');
+        }
+        /* eslint-enable require-yield */
+      }
+
+      const agent = new Agent({
+        name: 'BlockingGuard',
+        model: new ExpectGuardrailFirstModel(),
+        inputGuardrails: [blockingGuardrail],
+      });
+
+      const result = await run(agent, 'hello');
+      expect(result.finalOutput).toBe('done');
+      expect(result.inputGuardrailResults).toHaveLength(1);
+      expect(blockingGuardrail.execute).toHaveBeenCalledTimes(1);
+    });
+
     it('output guardrail success', async () => {
       const guardrailFn = vi.fn(async () => ({
         tripwireTriggered: false,