fix(tracing): preserve tool names in ChatML history

plugins/tracing/src/trace.ts

@@ -337,7 +337,7 @@ function assistantMessage(step: ModelStep): ChatMlMessage {
             id: tc.callId,
             type: "function" as const,
             function: {
-              name: toolObservationName(tc),
+              name: tc.name || "tool",
               ...(tc.args !== undefined ? { arguments: toText(tc.args) } : {}),
             },
           })),
@@ -350,7 +350,7 @@ function toolMessages(step: ModelStep): ChatMlMessage[] {
   return step.toolCalls.map((tc) => ({
     role: "tool" as const,
     tool_call_id: tc.callId,
-    name: toolObservationName(tc),
+    name: tc.name || "tool",
     content: tc.output != null ? toText(tc.output) : (tc.error ?? ""),
     ...(tc.error ? { is_error: true as const } : {}),
   }));

plugins/tracing/test/trace.test.ts

@@ -219,6 +219,40 @@ describe("convertRollout", () => {
     expect(meta("changed_this_turn")).toBe("true");
   });
 
+  it.each(["", undefined])("keeps the history fallback for tool name %j", async (name) => {
+    const dir = stageFixtures();
+    const file = path.join(dir, "rollout-basic-main.jsonl");
+    const lines = fs
+      .readFileSync(file, "utf8")
+      .trim()
+      .split("\n")
+      .map((line) => JSON.parse(line));
+    // Malformed rollouts still need usable names in both halves of the tool exchange.
+    lines.find((line) => line.payload.type === "function_call").payload.name = name;
+    fs.writeFileSync(file, lines.map((line) => JSON.stringify(line)).join("\n") + "\n");
+    await convertRollout(file, { config: baseConfig });
+
+    const generations = exporter
+      .getFinishedSpans()
+      .filter((s) => obsType(s) === "generation")
+      .sort((a, b) => startMs(a) - startMs(b));
+    const history = JSON.parse(attr(generations[1], "langfuse.observation.input"));
+    expect(history).toContainEqual(
+      expect.objectContaining({
+        role: "assistant",
+        tool_calls: [
+          expect.objectContaining({
+            id: "call-1",
+            function: expect.objectContaining({ name: "tool" }),
+          }),
+        ],
+      }),
+    );
+    expect(history).toContainEqual(
+      expect.objectContaining({ role: "tool", tool_call_id: "call-1", name: "tool" }),
+    );
+  });
+
   it("carries earlier turns of the thread into a later turn's generation input", async () => {
     const dir = fs.mkdtempSync(path.join(os.tmpdir(), "lf-codex-hist-"));
     const day = path.join(dir, "sessions", "2026", "06", "03");
@@ -783,7 +817,23 @@ describe("convertRollout", () => {
 
   it("captures web search, local shell, and MCP tool calls with specific names", async () => {
     const dir = stageFixtures();
-    await convertRollout(path.join(dir, "rollout-tools-main.jsonl"), { config: baseConfig });
+    const file = path.join(dir, "rollout-tools-main.jsonl");
+    const lines = fs
+      .readFileSync(file, "utf-8")
+      .trim()
+      .split("\n")
+      .map((line) => JSON.parse(line));
+    lines.splice(
+      lines.findIndex((line) => line.payload.role === "assistant"),
+      0,
+      {
+        timestamp: "2026-06-03T12:00:04.700Z",
+        type: "event_msg",
+        payload: { type: "token_count" },
+      },
+    );
+    fs.writeFileSync(file, lines.map((line) => JSON.stringify(line)).join("\n") + "\n");
+    await convertRollout(file, { config: baseConfig });
 
     const spans = exporter.getFinishedSpans();
     const toolNames = spans
@@ -798,6 +848,30 @@ describe("convertRollout", () => {
 
     const shell = spans.find((s) => s.name === "local_shell")!;
     expect(attr(shell, "langfuse.observation.output")).toContain("clean");
+
+    const generations = spans
+      .filter((s) => obsType(s) === "generation")
+      .sort((a, b) => startMs(a) - startMs(b));
+    expect(generations).toHaveLength(2);
+    const history = JSON.parse(attr(generations[1], "langfuse.observation.input"));
+    expect(history).toContainEqual(
+      expect.objectContaining({
+        role: "assistant",
+        tool_calls: expect.arrayContaining([
+          expect.objectContaining({
+            id: "call-mcp",
+            function: expect.objectContaining({ name: "linear__create_issue" }),
+          }),
+        ]),
+      }),
+    );
+    expect(history).toContainEqual(
+      expect.objectContaining({
+        role: "tool",
+        tool_call_id: "call-mcp",
+        name: "linear__create_issue",
+      }),
+    );
   });
 
   it("does not re-export a settings event between turns on every invocation", async () => {
@@ -975,6 +1049,38 @@ describe("deterministic trace ids (trace_seed)", () => {
 });
 
 describe("skill observations", () => {
+  it.each([true, false])(
+    "keeps real tool names in generation history with skill_tags=%s",
+    async (skill_tags) => {
+      const dir = stageFixtures();
+      await convertRollout(path.join(dir, "rollout-skills-main.jsonl"), {
+        config: { ...baseConfig, skill_tags },
+      });
+
+      const spans = exporter.getFinishedSpans();
+      const generations = spans
+        .filter((s) => obsType(s) === "generation")
+        .sort((a, b) => startMs(a) - startMs(b));
+      expect(generations).toHaveLength(2);
+      const history = JSON.parse(attr(generations[1], "langfuse.observation.input"));
+      expect(history).toContainEqual(
+        expect.objectContaining({
+          role: "assistant",
+          tool_calls: expect.arrayContaining([
+            expect.objectContaining({
+              id: "call-doc",
+              function: expect.objectContaining({ name: "exec_command" }),
+            }),
+          ]),
+        }),
+      );
+      expect(history).toContainEqual(
+        expect.objectContaining({ role: "tool", tool_call_id: "call-doc", name: "exec_command" }),
+      );
+      expect(spans.find((s) => s.name === "skill:bug-mentor")).toBeDefined();
+    },
+  );
+
   it("names a skill-loading command after the skill, keeping its own timing", async () => {
     const dir = stageFixtures();
     await convertRollout(path.join(dir, "rollout-skills-main.jsonl"), { config: baseConfig });