feat(custom-model): warn before launching Claude on a model too small for its own overhead

Claude Code's own fixed per-turn overhead (system prompt + tool schemas,
~36.4K tokens measured live) can exceed a small local model's entire real
context before any conversation history exists to compact — confirmed
live twice as an in:0 out:0 failure on the very first message sent.
CLAUDE_CODE_MAX_CONTEXT_TOKENS cannot fix this: it only governs when
history gets compacted, and there is none on message one.

- exceedsSafeContextFloor() (custom-model-routes.ts): true when a CLI's
  registry entry declares contextLengthVar (currently only claude) and
  the model's discovered context is below CLAUDE_MIN_SAFE_CONTEXT_TOKENS
  (40000). A no-op for every other CLI by construction.
- Both apply routes (POST /api/sessions/:id/custom-model and the
  quick-start customModel path) check this before the swap-conflict
  check and before launching/restarting anything, returning
  {requiresContextWarning, modelId, contextLength, minSafeContextTokens}
  — skipped when confirmed:true.
- Frontend: #customModelContextWarningModal + _confirmContextWarning/
  _resolveContextWarningConfirm (session-ui.js), wired into both
  _quickStartWithCustomModelConfirm and _runCustomModelEntryViaRestart
  (the path Claude actually uses) ahead of the swap-confirmation check.
  Explains the fix in-modal: give the model an explicit larger -c/
  --ctx-size in llama-swap instead of relying on --fit-ctx, which
  optimizes for the biggest model that fits rather than the biggest
  context.

Tests added for the route-level warning/confirm/skip cases and the
frontend modal + launch-flow wiring. Docs updated (custom-model-
endpoints.md, wiki/Custom-Model-Endpoints.md) and the PR's running
changeset extended.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

test/routes/quick-start-custom-model.test.ts

@@ -275,6 +275,59 @@ describe('POST /api/quick-start: customModel (one-shot custom-model launch)', ()
     });
   });
 
+  describe("context-window floor warning (this CLI's own overhead can exceed a small model's real context)", () => {
+    const SMALL_CTX_ENDPOINT: CustomModelHost = {
+      id: 'ep-small',
+      label: 'tiny box',
+      baseUrl: 'http://192.168.1.51:8080',
+      apiKey: 'k',
+      modelContextLengths: { 'qwen3.8-27b-ud-q4_k_xl': 16384 },
+    };
+
+    it('warns instead of launching when the discovered context is below the safe floor', async () => {
+      await writeCustomModelHosts(getDataDir(), [ENDPOINT, SMALL_CTX_ENDPOINT]);
+
+      const res = await quickStart({
+        caseName: 'cm-small-ctx',
+        mode: 'claude',
+        customModel: { endpointId: 'ep-small', modelId: 'qwen3.8-27b-ud-q4_k_xl' },
+      });
+
+      const body = res.json();
+      expect(body.requiresContextWarning).toBe(true);
+      expect(body.modelId).toBe('qwen3.8-27b-ud-q4_k_xl');
+      expect(body.contextLength).toBe(16384);
+      expect(body.minSafeContextTokens).toBe(40000);
+      // Nothing was actually created.
+      expect(ctx.sessions.size).toBe(1);
+    });
+
+    it('launches once confirmed, skipping the context check', async () => {
+      await writeCustomModelHosts(getDataDir(), [ENDPOINT, SMALL_CTX_ENDPOINT]);
+
+      const res = await quickStart({
+        caseName: 'cm-small-ctx-confirmed',
+        mode: 'claude',
+        customModel: { endpointId: 'ep-small', modelId: 'qwen3.8-27b-ud-q4_k_xl', confirmed: true },
+      });
+
+      expect(res.statusCode).toBe(200);
+      expect(res.json().requiresContextWarning).toBeUndefined();
+      expect(ctx.sessions.size).toBe(2);
+    });
+
+    it('does not warn when nothing about context was discovered', async () => {
+      const res = await quickStart({
+        caseName: 'cm-no-ctx-data',
+        mode: 'claude',
+        customModel: { endpointId: 'ep1', modelId: 'qwen3' },
+      });
+
+      expect(res.statusCode).toBe(200);
+      expect(res.json().requiresContextWarning).toBeUndefined();
+    });
+  });
+
   describe('triggering the actual llama-swap load (not just watching for it)', () => {
     it('sends a real inference request naming the target model, concurrently with launching the session', async () => {
       const chatCalls: unknown[] = [];