test: add comprehensive tests for tool_use/tool_result repair after truncation

Add tests for PR #4387 covering three critical scenarios:
1. Orphaned tool_use (tool call without result after truncation) - verifies synthetic error result insertion
2. Orphaned tool_result (result without call after truncation) - verifies orphan dropping
3. Normal history (well-formed transcript) - verifies no-op behavior

Also tests edge cases:
- Multiple orphaned tool_use blocks
- Mixed scenario with some results present, some missing

All tests pass. Fixes #4367

src/agents/session-transcript-repair.test.ts

@@ -109,4 +109,150 @@ describe("sanitizeToolUseResultPairing", () => {
     expect(out.some((m) => m.role === "toolResult")).toBe(false);
     expect(out.map((m) => m.role)).toEqual(["user", "assistant"]);
   });
+
+  // Tests for PR #4387: tool_use_id mismatch fix after history truncation
+  describe("after history truncation", () => {
+    it("repairs orphaned tool_use by inserting synthetic error result", () => {
+      // Simulates truncation that removed the tool_result but kept the assistant tool_use
+      const truncatedHistory = [
+        {
+          role: "assistant",
+          content: [{ type: "toolCall", id: "call_1", name: "read", arguments: {} }],
+        },
+        { role: "user", content: "what did you find?" },
+      ] satisfies AgentMessage[];
+
+      const out = sanitizeToolUseResultPairing(truncatedHistory);
+
+      // Should have inserted a synthetic error result after the assistant message
+      expect(out).toHaveLength(3);
+      expect(out[0]?.role).toBe("assistant");
+      expect(out[1]?.role).toBe("toolResult");
+      expect((out[1] as { toolCallId?: string }).toolCallId).toBe("call_1");
+      expect((out[1] as { isError?: boolean }).isError).toBe(true);
+      expect((out[1] as { content?: Array<{ text?: string }> }).content?.[0]?.text).toContain(
+        "missing tool result",
+      );
+      expect(out[2]?.role).toBe("user");
+    });
+
+    it("drops orphaned tool_result that no longer has matching tool_use", () => {
+      // Simulates truncation that removed the assistant tool_use but kept the tool_result
+      const truncatedHistory = [
+        { role: "user", content: "please read the file" },
+        {
+          role: "toolResult",
+          toolCallId: "call_orphan",
+          toolName: "read",
+          content: [{ type: "text", text: "file contents" }],
+          isError: false,
+        },
+        {
+          role: "assistant",
+          content: [{ type: "text", text: "Here's what I found..." }],
+        },
+      ] satisfies AgentMessage[];
+
+      const out = sanitizeToolUseResultPairing(truncatedHistory);
+
+      // Orphaned tool_result should be dropped
+      expect(out).toHaveLength(2);
+      expect(out.map((m) => m.role)).toEqual(["user", "assistant"]);
+      expect(out.some((m) => m.role === "toolResult")).toBe(false);
+    });
+
+    it("passes through normal history unchanged", () => {
+      // Well-formed history with proper tool_use/tool_result pairing
+      const normalHistory = [
+        { role: "user", content: "please read the file" },
+        {
+          role: "assistant",
+          content: [{ type: "toolCall", id: "call_1", name: "read", arguments: {} }],
+        },
+        {
+          role: "toolResult",
+          toolCallId: "call_1",
+          toolName: "read",
+          content: [{ type: "text", text: "file contents" }],
+          isError: false,
+        },
+        {
+          role: "assistant",
+          content: [{ type: "text", text: "Here's what I found..." }],
+        },
+        { role: "user", content: "thanks!" },
+      ] satisfies AgentMessage[];
+
+      const out = sanitizeToolUseResultPairing(normalHistory);
+
+      // Should return the same array reference (no modifications)
+      expect(out).toBe(normalHistory);
+      expect(out).toHaveLength(5);
+      expect(out.map((m) => m.role)).toEqual([
+        "user",
+        "assistant",
+        "toolResult",
+        "assistant",
+        "user",
+      ]);
+    });
+
+    it("handles multiple orphaned tool_use blocks after truncation", () => {
+      const truncatedHistory = [
+        {
+          role: "assistant",
+          content: [
+            { type: "toolCall", id: "call_1", name: "read", arguments: {} },
+            { type: "toolCall", id: "call_2", name: "exec", arguments: {} },
+          ],
+        },
+        { role: "user", content: "what happened?" },
+      ] satisfies AgentMessage[];
+
+      const out = sanitizeToolUseResultPairing(truncatedHistory);
+
+      // Should insert synthetic results for both orphaned tool calls
+      expect(out).toHaveLength(4);
+      expect(out[0]?.role).toBe("assistant");
+      expect(out[1]?.role).toBe("toolResult");
+      expect((out[1] as { toolCallId?: string }).toolCallId).toBe("call_1");
+      expect(out[2]?.role).toBe("toolResult");
+      expect((out[2] as { toolCallId?: string }).toolCallId).toBe("call_2");
+      expect(out[3]?.role).toBe("user");
+    });
+
+    it("repairs mixed scenario: some results present, some missing after truncation", () => {
+      const truncatedHistory = [
+        {
+          role: "assistant",
+          content: [
+            { type: "toolCall", id: "call_1", name: "read", arguments: {} },
+            { type: "toolCall", id: "call_2", name: "exec", arguments: {} },
+          ],
+        },
+        {
+          role: "toolResult",
+          toolCallId: "call_1",
+          toolName: "read",
+          content: [{ type: "text", text: "contents" }],
+          isError: false,
+        },
+        // call_2's result was truncated away
+        { role: "user", content: "ok" },
+      ] satisfies AgentMessage[];
+
+      const out = sanitizeToolUseResultPairing(truncatedHistory);
+
+      // Should keep existing result and insert synthetic result for missing one
+      expect(out).toHaveLength(4);
+      expect(out[0]?.role).toBe("assistant");
+      expect(out[1]?.role).toBe("toolResult");
+      expect((out[1] as { toolCallId?: string }).toolCallId).toBe("call_1");
+      expect((out[1] as { isError?: boolean }).isError).toBe(false);
+      expect(out[2]?.role).toBe("toolResult");
+      expect((out[2] as { toolCallId?: string }).toolCallId).toBe("call_2");
+      expect((out[2] as { isError?: boolean }).isError).toBe(true);
+      expect(out[3]?.role).toBe("user");
+    });
+  });
 });