fix(models): generateStream capability-mismatch records the resolved backend name

Caught by Claude review bot on PR #638: `generateStream`'s catch was
hard-coding `undefined` as the backend, so capability-mismatch
failures (registry hit, capability check fail) produced
`backend: 'unknown'` rows even though the backend WAS found —
diverging from `embed` / `generate` which correctly pass the
captured `backend` variable. The F2 fix in commit fabef73 covered
the registry-miss case but missed the capability-mismatch path on
streams specifically.

Change: declare `let backend: ModelBackend | undefined` (matches
embed/generate) and pass `backend` (not `undefined`) into
`#recordFailure`. When `resolveGenerative()` throws (registry miss),
`backend` is still undefined and the row records `backend: 'unknown'`
as before. When `requireCapability()` throws, `backend` is the
resolved instance and the row records its actual name.

Test: new "records capability-mismatch on generateStream with the
resolved backend name" exercises a backend registered with
`stream: false` and asserts `backend: 'no-stream'` (not 'unknown')
+ `error_code: 'capability_unsupported'`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: heskew

resources/models/Models.ts

@@ -81,14 +81,17 @@ export class Models implements ModelsContract {
 	generateStream(input: GenerateInput, opts: GenerateOpts = {}): AsyncIterable<GenerateChunk> {
 		const { accounting, signal } = resolveCallContext(opts.signal);
 		const startedAt = performance.now();
-		let backend: ModelBackend;
+		let backend: ModelBackend | undefined;
 		try {
 			backend = resolveGenerative(opts.model);
 			requireCapability(backend, 'stream');
 		} catch (err) {
 			// Record pre-call failure synchronously so callers that hold but never
 			// iterate the returned iterable still produce a billing row, then rethrow.
-			this.#recordFailure(undefined, 'generateStream', opts.model, accounting, opts, startedAt, err);
+			// Pass `backend` (not `undefined`): when the registry hit but the capability
+			// check failed, `backend` is the resolved instance and its name belongs in
+			// the row. Only registry misses leave `backend` undefined → 'unknown'.
+			this.#recordFailure(backend, 'generateStream', opts.model, accounting, opts, startedAt, err);
 			throw err;
 		}
 		const backendOpts: BackendOpts<GenerateOpts> = { ...opts, signal, accounting };

unitTests/resources/models/Models.test.js

@@ -275,5 +275,22 @@ describe('Models facade', () => {
 			assert.strictEqual(writer.records[0].error_code, 'backend_not_found');
 			assert.strictEqual(writer.records[0].method, 'generateStream');
 		});
+
+		it('records capability-mismatch on generateStream with the resolved backend name (not "unknown")', async () => {
+			setGenerative('default', {
+				name: 'no-stream',
+				capabilities: () => ({ embed: false, generate: true, stream: false, tools: false, adapters: false }),
+			});
+			await assert.rejects(async () => {
+				// eslint-disable-next-line no-unused-vars
+				for await (const _ of models.generateStream('x')) {
+					// will not iterate — requireCapability throws first
+				}
+			});
+			assert.strictEqual(writer.records.length, 1);
+			assert.strictEqual(writer.records[0].backend, 'no-stream');
+			assert.strictEqual(writer.records[0].error_code, 'capability_unsupported');
+			assert.strictEqual(writer.records[0].method, 'generateStream');
+		});
 	});
 });