Re-carry the claude-fable-5-1 adaptive thinking patch lost in the vendor sync

The 2026-09-07 re-sync to upstream 0205b07b dropped one divergence that
was never ledgered: claude-fable-5-1 in anthropic.ts's
ADAPTIVE_THINKING_MODELS, plus the adaptive-thinking request-shape suite
in anthropic.test.ts. The pristine sync restored upstream's list, so the
shipped model would receive the thinking:{type:enabled,budget_tokens}
shape that adaptive-only models reject. Restore both in the patch layer,
ledger the divergence as providers-ts-anthropic-adaptive-fable-5-1, and
correct the re-sync narrative so it records the drop and recovery.

Author: TheGreatAxios

vendor/intx-inference/PATCHES.md

@@ -23,7 +23,12 @@ the new upstream code, not by assuming).
 ### 2026-09-07 re-sync (upstream `0205b07b`)
 
 Every entry above was re-carried against the new pin; none was dropped as
-upstream-absorbed. Upstream changes in the pinned range touched exactly two
+upstream-absorbed. The sync also dropped one divergence that was **not**
+ledgered at the time: `claude-fable-5-1` in `providers/anthropic.ts`'s
+`ADAPTIVE_THINKING_MODELS` and its "adaptive thinking request shape" suite in
+`providers/anthropic.test.ts`. It is recovered post-sync as
+`providers-ts-anthropic-adaptive-fable-5-1` below. Upstream changes in the
+pinned range touched exactly two
 patched files: `reactor.ts` (doom-loop detection: `doomLoopThreshold`
 config, `toolBatchSignature`, run-scoped repeat accounting in `executeTools`,
 and a fatal break in the action loop) and `assembly.ts` (a
@@ -253,6 +258,23 @@ gap rather than carrying indefinitely.
 Corbits adds a local type assertion wrapper and removes the cast from the
 vendored patch.
 
+## providers-ts-anthropic-adaptive-fable-5-1
+
+`providers/anthropic.ts` — Adds `claude-fable-5-1` to
+`ADAPTIVE_THINKING_MODELS`, so the adapter sends the
+`thinking:{type:"adaptive"}` + `output_config.effort` wire shape the model
+requires instead of the `thinking:{type:"enabled",budget_tokens}` shape
+adaptive-only models reject. Upstream's list lacks the model. Consumed by the
+`first-class-providers` registry (`claude-fable-5-1` is a shipped, selectable
+anthropic and zen model; CHANGELOG 0.3.17 advertises adaptive thinking for
+Fable 5) and guarded by the "adaptive thinking request shape" suite in
+`providers/anthropic.test.ts`.
+
+**Disposition:** Re-carryable — a one-line list addition that survives sync
+trivially; the guard suite re-applies verbatim. Risk: upstream may grow its
+own adaptive-models list; reconcile the two on next sync. **Removal path:**
+Upstream adding `claude-fable-5-1` to its own `ADAPTIVE_THINKING_MODELS`.
+
 ---
 
 The `void track(p)` → `track(p)` change at three call sites in `reactor.ts`

vendor/intx-inference/src/providers/anthropic.test.ts

@@ -817,6 +817,47 @@ describe("Anthropic adapter — responseFormat boundary", () => {
   });
 });
 
+describe("Anthropic adapter — adaptive thinking request shape", () => {
+  const ThinkingBody = type({
+    thinking: {
+      type: "string",
+      "budget_tokens?": "number",
+    },
+    "output_config?": { effort: "string" },
+  });
+
+  function parseThinkingBody(body: string) {
+    const parsed = ThinkingBody(JSON.parse(body));
+    if (parsed instanceof type.errors) {
+      throw new Error(`unexpected request body shape: ${parsed.summary}`);
+    }
+    return parsed;
+  }
+
+  test("claude-fable-5-1 with thinking.enabled uses type adaptive and output_config.effort", () => {
+    const req = createAnthropicAdapter(TEST_SOURCE).buildRequest(
+      [],
+      "claude-fable-5-1",
+      { thinking: { enabled: true } },
+    );
+    const body = parseThinkingBody(req.body);
+    expect(body.thinking).toEqual({ type: "adaptive" });
+    expect(body.output_config?.effort).toBeDefined();
+  });
+
+  test("a non-adaptive model with thinking.enabled uses type enabled and budget_tokens", () => {
+    const req = createAnthropicAdapter(TEST_SOURCE).buildRequest(
+      [],
+      "claude-haiku-4-5",
+      { thinking: { enabled: true } },
+    );
+    const body = parseThinkingBody(req.body);
+    expect(body.thinking.type).toBe("enabled");
+    expect(body.thinking.budget_tokens).toBeDefined();
+    expect(body.output_config).toBeUndefined();
+  });
+});
+
 describe("Anthropic adapter — tool-name codec round-trip", () => {
   const PREFIXED = "@intx/tools-posix/sidecar-bundle:run_shell";
   const ToolsBody = type({ tools: type({ name: "string" }).array() });

vendor/intx-inference/src/providers/anthropic.ts

@@ -39,6 +39,8 @@ export const ADAPTIVE_THINKING_MODELS: ReadonlySet<string> = new Set([
   "claude-sonnet-5",
   "claude-opus-5",
   "claude-fable-5",
+  // Locally patched — see vendor/intx-inference/PATCHES.md#providers-ts-anthropic-adaptive-fable-5-1
+  "claude-fable-5-1",
   "claude-opus-4-8",
   "claude-opus-4-6",
   "claude-opus-4-7",