feat(skl,cml): --context-window auto model→window probe (v5.12 B8b) [skip-docs]

Completes the deferred B8 half. `--context-window auto` now probes the
configured model and calibrates SKL/CML budgets to its real window instead
of always falling back to the conservative advisory anchor.

- lib/context-window.mjs: pure modelToContextWindow() maps known 1M-tier
  model IDs (Fable 5, Opus 4.8/4.7/4.6, Sonnet 4.6 — verified June 2026 —
  plus the explicit [1m] tier tag, dated/provider-prefixed IDs, and the
  opus/sonnet/fable aliases) to the 1M window; unknown/unconfirmed -> null
  (caller keeps the conservative anchor). resolveContextWindow() auto branch
  now probes opts.model: recognized -> auto-probed (not advisory); unknown
  or unpinned -> auto-unresolved (advisory, pre-B8b behavior).
- lib/active-model.mjs (new): resolveActiveModel() reads the model the way
  Claude Code resolves it — shell ANTHROPIC_MODEL override, then settings
  cascade local > project > user. Injectable env, hermetic under test HOME.
- scan-orchestrator: resolves the active model only when the flag is `auto`
  and threads it into resolveContextWindow; posture inherits via runAllScanners.

Default (no flag) and explicit --context-window <n> paths ignore the model
and stay byte-stable; frozen v5.0.0 + SC-5 snapshots untouched. TDD: 17 new
tests (context-window mapping/probe + active-model cascade). Suite 1296 green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/lib/context-window.test.mjs

@@ -3,6 +3,7 @@ import assert from 'node:assert/strict';
 import {
   CONTEXT_WINDOW_ANCHOR,
   LARGE_CONTEXT_WINDOW,
+  modelToContextWindow,
   resolveContextWindow,
   scaleForWindow,
 } from '../../scanners/lib/context-window.mjs';
@@ -43,13 +44,20 @@ describe('resolveContextWindow — explicit window', () => {
 });
 
 describe('resolveContextWindow — auto / unknown downgrades to advisory', () => {
-  it('"auto" keeps the conservative anchor but flags advisory (no model probe yet)', () => {
+  it('"auto" with no model keeps the conservative anchor but flags advisory', () => {
     const r = resolveContextWindow('auto');
     assert.equal(r.window, CONTEXT_WINDOW_ANCHOR, 'window stays the conservative anchor');
     assert.equal(r.advisory, true, 'unknown window -> advisory, not a budget breach');
     assert.equal(r.source, 'auto-unresolved');
   });
 
+  it('"auto" with an unrecognized model also stays advisory', () => {
+    const r = resolveContextWindow('auto', { model: 'totally-made-up-model' });
+    assert.equal(r.window, CONTEXT_WINDOW_ANCHOR);
+    assert.equal(r.advisory, true);
+    assert.equal(r.source, 'auto-unresolved');
+  });
+
   it('"AUTO" is case-insensitive', () => {
     assert.equal(resolveContextWindow('AUTO').advisory, true);
   });
@@ -74,3 +82,86 @@ describe('scaleForWindow — linear scaling off the 200k anchor', () => {
     assert.equal(scaleForWindow(40_000, LARGE_CONTEXT_WINDOW), 200_000);
   });
 });
+
+// B8b — model -> context-window mapping. `--context-window auto` probes the
+// configured model and maps known 1M-tier model IDs to the large window so SKL/CML
+// self-calibrate instead of falling back to the conservative advisory anchor.
+// Verified June 2026 (platform.claude.com models overview): Fable 5, Opus
+// 4.8/4.7/4.6 and Sonnet 4.6 run a 1M context window.
+
+describe('modelToContextWindow — known 1M-tier models map to the large window', () => {
+  it('the explicit [1m] tier tag wins (the running session model surfaces this way)', () => {
+    assert.equal(modelToContextWindow('claude-opus-4-8[1m]'), LARGE_CONTEXT_WINDOW);
+    // Suffix wins even for a base ID we do not otherwise enumerate.
+    assert.equal(modelToContextWindow('claude-future-9[1m]'), LARGE_CONTEXT_WINDOW);
+  });
+
+  it('known 1M base IDs map to 1M', () => {
+    for (const id of [
+      'claude-fable-5',
+      'claude-opus-4-8',
+      'claude-opus-4-7',
+      'claude-opus-4-6',
+      'claude-sonnet-4-6',
+    ]) {
+      assert.equal(modelToContextWindow(id), LARGE_CONTEXT_WINDOW, id);
+    }
+  });
+
+  it('tolerates dated suffixes and provider prefixes (substring match)', () => {
+    assert.equal(modelToContextWindow('claude-opus-4-8-20260528'), LARGE_CONTEXT_WINDOW);
+    assert.equal(modelToContextWindow('us.anthropic.claude-sonnet-4-6'), LARGE_CONTEXT_WINDOW);
+    assert.equal(modelToContextWindow('CLAUDE-OPUS-4-8'), LARGE_CONTEXT_WINDOW); // case-insensitive
+  });
+
+  it('short aliases that resolve to a 1M-tier model map to 1M', () => {
+    for (const alias of ['opus', 'sonnet', 'fable', 'opusplan']) {
+      assert.equal(modelToContextWindow(alias), LARGE_CONTEXT_WINDOW, alias);
+    }
+  });
+});
+
+describe('modelToContextWindow — unknown / unconfirmed models return null (conservative)', () => {
+  it('returns null for models whose window we do not confirm', () => {
+    // Haiku is intentionally NOT enumerated: keep the conservative anchor rather
+    // than guess. null -> caller keeps the advisory 200k anchor.
+    assert.equal(modelToContextWindow('claude-haiku-4-5-20251001'), null);
+    assert.equal(modelToContextWindow('haiku'), null);
+    assert.equal(modelToContextWindow('some-unknown-model'), null);
+    assert.equal(modelToContextWindow('default'), null);
+  });
+
+  it('returns null for non-string / empty input', () => {
+    assert.equal(modelToContextWindow(undefined), null);
+    assert.equal(modelToContextWindow(null), null);
+    assert.equal(modelToContextWindow(''), null);
+    assert.equal(modelToContextWindow('   '), null);
+    assert.equal(modelToContextWindow(42), null);
+  });
+});
+
+describe('resolveContextWindow — auto probes the model (B8b)', () => {
+  it('"auto" + a recognized 1M model calibrates to 1M, not advisory', () => {
+    const r = resolveContextWindow('auto', { model: 'claude-opus-4-8[1m]' });
+    assert.equal(r.window, LARGE_CONTEXT_WINDOW);
+    assert.equal(r.advisory, false, 'a confirmed window is not advisory');
+    assert.equal(r.source, 'auto-probed');
+  });
+
+  it('"auto" + a recognized base ID (no tag) also calibrates', () => {
+    const r = resolveContextWindow('auto', { model: 'claude-sonnet-4-6' });
+    assert.equal(r.window, LARGE_CONTEXT_WINDOW);
+    assert.equal(r.source, 'auto-probed');
+  });
+
+  it('the model is ignored on the explicit and default paths (byte-stable)', () => {
+    const explicit = resolveContextWindow('1000000', { model: 'claude-opus-4-8' });
+    assert.equal(explicit.source, 'explicit');
+    assert.equal(explicit.window, 1_000_000);
+
+    const def = resolveContextWindow(undefined, { model: 'claude-opus-4-8[1m]' });
+    assert.equal(def.source, 'default');
+    assert.equal(def.window, CONTEXT_WINDOW_ANCHOR);
+    assert.equal(def.advisory, false);
+  });
+});