diff --git a/src/agents/session-tool-result-guard-wrapper.ts b/src/agents/session-tool-result-guard-wrapper.ts
index 956247a24..fcc0ef3b3 100644
--- a/src/agents/session-tool-result-guard-wrapper.ts
+++ b/src/agents/session-tool-result-guard-wrapper.ts
@@ -18,6 +18,7 @@ export function guardSessionManager(
     agentId?: string;
     sessionKey?: string;
     allowSyntheticToolResults?: boolean;
+    dropOrphanToolResults?: boolean;
   },
 ): GuardedSessionManager {
   if (typeof (sessionManager as GuardedSessionManager).flushPendingToolResults === "function") {
@@ -48,6 +49,7 @@ export function guardSessionManager(
   const guard = installSessionToolResultGuard(sessionManager, {
     transformToolResultForPersistence: transform,
     allowSyntheticToolResults: opts?.allowSyntheticToolResults,
+    dropOrphanToolResults: opts?.dropOrphanToolResults,
   });
   (sessionManager as GuardedSessionManager).flushPendingToolResults = guard.flushPendingToolResults;
   return sessionManager as GuardedSessionManager;
diff --git a/src/agents/session-tool-result-guard.test.ts b/src/agents/session-tool-result-guard.test.ts
index 1bfcb31ed..def38a7ac 100644
--- a/src/agents/session-tool-result-guard.test.ts
+++ b/src/agents/session-tool-result-guard.test.ts
@@ -72,6 +72,25 @@ describe("installSessionToolResultGuard", () => {
     expect(messages.map((m) => m.role)).toEqual(["assistant", "toolResult"]);
   });
 
+  it("drops orphan toolResult messages by default", () => {
+    const sm = SessionManager.inMemory();
+    installSessionToolResultGuard(sm);
+
+    sm.appendMessage({
+      role: "toolResult",
+      toolCallId: "orphan_call",
+      content: [{ type: "text", text: "no matching tool call" }],
+      isError: true,
+    } as AgentMessage);
+
+    const messages = sm
+      .getEntries()
+      .filter((e) => e.type === "message")
+      .map((e) => (e as { message: AgentMessage }).message);
+
+    expect(messages).toHaveLength(0);
+  });
+
   it("preserves ordering with multiple tool calls and partial results", () => {
     const sm = SessionManager.inMemory();
     const guard = installSessionToolResultGuard(sm);
diff --git a/src/agents/session-tool-result-guard.ts b/src/agents/session-tool-result-guard.ts
index feb6b854c..6cfc8aeb0 100644
--- a/src/agents/session-tool-result-guard.ts
+++ b/src/agents/session-tool-result-guard.ts
@@ -49,6 +49,11 @@ export function installSessionToolResultGuard(
      * Defaults to true.
      */
     allowSyntheticToolResults?: boolean;
+    /**
+     * Whether to drop toolResult messages that do not match a pending tool call.
+     * Defaults to true to avoid corrupting transcripts with orphan tool results.
+     */
+    dropOrphanToolResults?: boolean;
   },
 ): {
   flushPendingToolResults: () => void;
@@ -66,6 +71,7 @@ export function installSessionToolResultGuard(
   };
 
   const allowSyntheticToolResults = opts?.allowSyntheticToolResults ?? true;
+  const dropOrphanToolResults = opts?.dropOrphanToolResults ?? true;
 
   const flushPendingToolResults = () => {
     if (pending.size === 0) return;
@@ -90,6 +96,9 @@ export function installSessionToolResultGuard(
     if (role === "toolResult") {
       const id = extractToolResultId(message as Extract<AgentMessage, { role: "toolResult" }>);
       const toolName = id ? pending.get(id) : undefined;
+      if (id && !pending.has(id) && dropOrphanToolResults) {
+        return undefined as never;
+      }
       if (id) pending.delete(id);
       return originalAppend(
         persistToolResult(message, {