fix(llm): preserve tool error defects (#27403)

packages/llm/src/schema/errors.ts

@@ -198,5 +198,6 @@ export class LLMError extends Schema.TaggedErrorClass<LLMError>()("LLM.Error", {
  */
 export class ToolFailure extends Schema.TaggedErrorClass<ToolFailure>()("LLM.ToolFailure", {
   message: Schema.String,
+  error: Schema.optional(Schema.Defect),
   metadata: Schema.optional(Schema.Record(Schema.String, Schema.Unknown)),
 }) {}

packages/llm/src/schema/events.ts

@@ -171,6 +171,7 @@ export const ToolError = Schema.Struct({
   id: ToolCallID,
   name: Schema.String,
   message: Schema.String,
+  error: Schema.optional(Schema.Defect),
   providerMetadata: Schema.optional(ProviderMetadata),
 }).annotate({ identifier: "LLM.Event.ToolError" })
 export type ToolError = Schema.Schema.Type<typeof ToolError>

packages/llm/src/tool-runtime.ts

@@ -112,17 +112,29 @@ export const stream = <T extends Tools>(options: StreamOptions<T>): Stream.Strea
 
             const dispatched = yield* Effect.forEach(
               state.toolCalls,
-              (call) => dispatch(tools, call).pipe(Effect.map((result) => [call, result] as const)),
+              (call) =>
+                dispatch(tools, call).pipe(Effect.map((result) => [call, result.result, result.error] as const)),
               { concurrency },
             )
-            const resultStream = Stream.fromIterable(dispatched.flatMap(([call, result]) => emitEvents(call, result)))
+            const resultStream = Stream.fromIterable(
+              dispatched.flatMap(([call, result, error]) => emitEvents(call, result, error)),
+            )
 
             if (!options.stopWhen) return resultStream.pipe(Stream.concat(finishStream))
             if (options.stopWhen({ step, request })) return resultStream.pipe(Stream.concat(finishStream))
 
             return resultStream.pipe(
               Stream.concat(
-                loop(followUpRequest(request, state, dispatched), step + 1, totalUsage, totalProviderMetadata),
+                loop(
+                  followUpRequest(
+                    request,
+                    state,
+                    dispatched.map(([call, result]) => [call, result] as const),
+                  ),
+                  step + 1,
+                  totalUsage,
+                  totalProviderMetadata,
+                ),
               ),
             )
           }),
@@ -215,7 +227,7 @@ const addUsage = (left: Usage | undefined, right: Usage | undefined) => {
     | "reasoningTokens"
     | "totalTokens"
   const sum = (key: UsageKey) =>
-    left[key] === undefined && right[key] === undefined ? undefined : Number(left[key] ?? 0) + Number(right[key] ?? 0)
+    left[key] === undefined && right[key] === undefined ? undefined : (left[key] ?? 0) + (right[key] ?? 0)
 
   return new Usage({
     inputTokens: sum("inputTokens"),
@@ -264,16 +276,20 @@ const appendStreamingText = (
   state.assistantContent.push({ type, text, providerMetadata })
 }
 
-const dispatch = (tools: Tools, call: ToolCallPart): Effect.Effect<ToolResultValue> => {
+const dispatch = (tools: Tools, call: ToolCallPart): Effect.Effect<{ result: ToolResultValue; error?: unknown }> => {
   const tool = tools[call.name]
-  if (!tool) return Effect.succeed({ type: "error" as const, value: `Unknown tool: ${call.name}` })
+  if (!tool) return Effect.succeed({ result: { type: "error" as const, value: `Unknown tool: ${call.name}` } })
   if (!tool.execute)
-    return Effect.succeed({ type: "error" as const, value: `Tool has no execute handler: ${call.name}` })
+    return Effect.succeed({ result: { type: "error" as const, value: `Tool has no execute handler: ${call.name}` } })
 
   return decodeAndExecute(tool, call).pipe(
     Effect.catchTag("LLM.ToolFailure", (failure) =>
-      Effect.succeed({ type: "error" as const, value: failure.message } satisfies ToolResultValue),
+      Effect.succeed({
+        result: { type: "error" as const, value: failure.message } satisfies ToolResultValue,
+        error: failure.error,
+      }),
     ),
+    Effect.map((result) => ("result" in result ? result : { result })),
   )
 }
 
@@ -294,10 +310,10 @@ const decodeAndExecute = (tool: AnyTool, call: ToolCallPart): Effect.Effect<Tool
     Effect.map((encoded): ToolResultValue => ({ type: "json", value: encoded })),
   )
 
-const emitEvents = (call: ToolCallPart, result: ToolResultValue): ReadonlyArray<LLMEvent> =>
+const emitEvents = (call: ToolCallPart, result: ToolResultValue, error: unknown): ReadonlyArray<LLMEvent> =>
   result.type === "error"
     ? [
-        LLMEvent.toolError({ id: call.id, name: call.name, message: String(result.value) }),
+        LLMEvent.toolError({ id: call.id, name: call.name, message: String(result.value), error }),
         LLMEvent.toolResult({ id: call.id, name: call.name, result }),
       ]
     : [LLMEvent.toolResult({ id: call.id, name: call.name, result })]

packages/llm/test/tool-runtime.test.ts

@@ -25,6 +25,7 @@ const baseRequest = LLM.request({
   model,
   prompt: "Use the tool.",
 })
+const weatherFailureCause = new Error("weather lookup denied")
 
 const get_weather = tool({
   description: "Get current weather for a city.",
@@ -32,7 +33,8 @@ const get_weather = tool({
   success: Schema.Struct({ temperature: Schema.Number, condition: Schema.String }),
   execute: ({ city }) =>
     Effect.gen(function* () {
-      if (city === "FAIL") return yield* new ToolFailure({ message: `Weather lookup failed for ${city}` })
+      if (city === "FAIL")
+        return yield* new ToolFailure({ message: `Weather lookup failed for ${city}`, error: weatherFailureCause })
       return { temperature: 22, condition: "sunny" }
     }),
 })
@@ -85,23 +87,27 @@ describe("LLMClient tools", () => {
         tools: { get_weather },
       }).pipe(Stream.runCollect, Effect.provide(layer))
 
-      const second = bodies[1] as {
+      const second = bodies[1]
-        readonly messages?: ReadonlyArray<Record<string, unknown>>
+      if (!second || typeof second !== "object") throw new Error("Expected second request body")
-        readonly tools?: ReadonlyArray<unknown>
+      const messages = Reflect.get(second, "messages")
-        readonly tool_choice?: unknown
+      const tools = Reflect.get(second, "tools")
-        readonly max_tokens?: unknown
-      }
 
-      expect(second.max_tokens).toBe(50)
+      expect(Reflect.get(second, "max_tokens")).toBe(50)
-      expect(second.tool_choice).toBe("auto")
+      expect(Reflect.get(second, "tool_choice")).toBe("auto")
-      expect(second.tools).toHaveLength(1)
+      expect(tools).toHaveLength(1)
-      expect(second.messages?.map((message) => message.role)).toEqual(["user", "assistant", "tool"])
+      expect(
-      expect(second.messages?.[1]).toMatchObject({
+        Array.isArray(messages)
+          ? messages.map((message) =>
+              message && typeof message === "object" ? Reflect.get(message, "role") : undefined,
+            )
+          : undefined,
+      ).toEqual(["user", "assistant", "tool"])
+      expect(Array.isArray(messages) ? messages[1] : undefined).toMatchObject({
         role: "assistant",
         content: null,
         tool_calls: [{ id: "call_1", type: "function", function: { name: "get_weather" } }],
       })
-      expect(second.messages?.[2]).toMatchObject({
+      expect(Array.isArray(messages) ? messages[2] : undefined).toMatchObject({
         role: "tool",
         tool_call_id: "call_1",
         content: '{"temperature":22,"condition":"sunny"}',
@@ -327,6 +333,7 @@ describe("LLMClient tools", () => {
       const toolError = events.find(LLMEvent.is.toolError)
       expect(toolError).toMatchObject({ type: "tool-error", id: "call_1", name: "get_weather" })
       expect(toolError?.message).toBe("Weather lookup failed for FAIL")
+      expect(toolError?.error).toBe(weatherFailureCause)
     }),
   )