fix(pi): say why text generation cannot load a model

packages/provider-pi/src/server/textGeneration.test.ts

@@ -14,7 +14,7 @@ import { makePiTextGeneration } from "./textGeneration.ts";
 const decodeJsonLine = Schema.decodeUnknownSync(Schema.fromJsonString(Schema.Unknown));
 const encodeJsonLine = Schema.encodeSync(Schema.fromJsonString(Schema.Unknown));
 
-const makeFakePi = (reply: string) =>
+const makeFakePi = (reply: string, setModelError?: string) =>
   Effect.gen(function* () {
     const stdout = yield* Queue.unbounded<Uint8Array, Cause.Done>();
     const received: Array<Record<string, unknown>> = [];
@@ -35,6 +35,8 @@ const makeFakePi = (reply: string) =>
             yield* emit({ type: "agent_settled" });
           } else if (record["type"] === "get_last_assistant_text") {
             yield* emit({ ...response, success: true, data: { text: reply } });
+          } else if (record["type"] === "set_model" && setModelError !== undefined) {
+            yield* emit({ ...response, success: false, error: setModelError });
           } else {
             yield* emit({ ...response, success: true });
           }
@@ -83,3 +85,29 @@ it.effect("puts linked source control context in the Pi thread title prompt", ()
     assert.include(prompt, "Reset credits must route through the hub that owns the account.");
   }),
 );
+
+it.effect("names the model and the disabled extensions when Pi cannot load it", () =>
+  Effect.gen(function* () {
+    const pi = yield* makeFakePi("", "Model not found: demo/demo-model");
+    const textGeneration = yield* makePiTextGeneration(
+      { enabled: true, binaryPath: "pi", launchArgs: "", customModels: [] },
+      {},
+    ).pipe(Effect.provideService(ChildProcessSpawner.ChildProcessSpawner, pi.spawner));
+
+    const error = yield* textGeneration
+      .generateCommitMessage({
+        cwd: process.cwd(),
+        branch: "main",
+        stagedSummary: "M README.md",
+        stagedPatch: "diff --git a/README.md b/README.md",
+        modelSelection: createModelSelection(ProviderInstanceId.make("pi"), "demo/demo-model"),
+      })
+      .pipe(Effect.flip);
+
+    assert.equal(error._tag, "TextGenerationError");
+    assert.include(error.detail, "Pi rejected model 'demo/demo-model'");
+    assert.include(error.detail, "Model not found: demo/demo-model");
+    assert.include(error.detail, "--no-extensions");
+    assert.isUndefined(pi.received.find((record) => record["type"] === "prompt"));
+  }),
+);

packages/provider-pi/src/server/textGeneration.ts

@@ -60,11 +60,24 @@ export const makePiTextGeneration = Effect.fn("makePiTextGeneration")(function*
             detail: `Pi model '${modelSelection.model}' must use provider/model format.`,
           });
         }
-        yield* connection.request({
-          type: "set_model",
-          provider: parsed.provider,
-          modelId: parsed.modelId,
-        });
+        yield* connection
+          .request({ type: "set_model", provider: parsed.provider, modelId: parsed.modelId })
+          .pipe(
+            // Model discovery loads extensions that this background helper disables.
+            Effect.mapError((error) =>
+              error._tag === "PiRpcError" && error.operation === "set_model"
+                ? new TextGenerationError({
+                    operation,
+                    detail:
+                      `Pi rejected model '${modelSelection.model}': ${error.detail ?? "unknown error"}. ` +
+                      "If the model comes from a Pi extension or package, text generation cannot use it " +
+                      "because it runs Pi with --no-extensions. Pick a built-in Pi model or one defined " +
+                      "in Pi's models.json.",
+                    cause: error,
+                  })
+                : error,
+            ),
+          );
       }
 
       yield* connection.request({ type: "prompt", message: prompt });