fix(custom-model): pi is told the endpoint's context window

A pi session on a custom endpoint got a model list entry with only the
model id. pi then takes the model for a 128,000-token one (its footer
reads "/128k") and asks for 16,384 reply tokens, so on a 65,536 or
32,768 window it never compacts before the server's real window is full.

The entry now carries `contextWindow` (the full window) and `maxTokens`
(a quarter of it, 8,192 at most) whenever the endpoint's context length
is known: the same two figures the DeepSeek Harness patch already
carries, now from one helper. An unknown length writes neither, as
before.

Measured on pi 1.5.2: with both fields `pi --list-models` shows the
window, and the request carries `max_tokens: 8192`.

Co-Authored-By: Claude Fable 5.2 <noreply@anthropic.com>

src/custom-model-injection.ts

@@ -194,6 +194,21 @@ function opencodeModelLimit(contextLength: number | undefined): { context: numbe
   return { context: total, output };
 }
 
+/**
+ * The two fields pi's model list and the dsh provider patch (which is built on pi's model
+ * layer) take for a model's window: `contextWindow`, the FULL window, and `maxTokens`, the
+ * reply's share of it (a quarter, capped at 8192 on a large window). Empty for an unknown or
+ * non-finite size: the CLI then keeps its own default, and no limit is invented here.
+ */
+function modelWindowFields(contextLength: number | undefined): { contextWindow?: number; maxTokens?: number } {
+  if (contextLength === undefined || !Number.isFinite(contextLength) || contextLength <= 0) return {};
+  return {
+    contextWindow: Math.trunc(contextLength),
+    // A quarter of the window keeps the reply inside the prompt's share; 8192 caps it on a large window.
+    maxTokens: Math.max(1, Math.min(8192, Math.floor(contextLength / 4))),
+  };
+}
+
 function renderConfigContent(
   template: 'opencode-json',
   endpoint: CustomModelEndpoint,
@@ -260,19 +275,7 @@ function renderConfigFile(
                     apiKeyEnv: CUSTOM_MODEL_API_KEY_ENV_VAR,
                     api: 'openai-completions',
                     baseURL: baseUrl,
-                    models: [
-                      {
-                        id: modelId,
-                        name: modelId,
-                        ...(contextLength !== undefined && Number.isFinite(contextLength) && contextLength > 0
-                          ? {
-                              contextWindow: Math.trunc(contextLength),
-                              // A quarter of the window keeps the reply inside the prompt's share; 8192 caps it on a large window.
-                              maxTokens: Math.max(1, Math.min(8192, Math.floor(contextLength / 4))),
-                            }
-                          : {}),
-                      },
-                    ],
+                    models: [{ id: modelId, name: modelId, ...modelWindowFields(contextLength) }],
                   },
                 },
               },
@@ -327,6 +330,11 @@ function renderConfigFile(
       // true` is required too: pi does not automatically send `Authorization: Bearer
       // <apiKey>` just because `apiKey` is set (per the same doc) — without it, a real
       // (non-llama.cpp) endpoint that actually checks the key would reject every request.
+      // The model entry carries the endpoint's context window when it is known. Without it pi
+      // takes an unknown model for a 128,000-token one (its footer reads `/128k`) and asks for
+      // 16,384 reply tokens, so on a 65,536 or 32,768 window it never compacts before the
+      // server's real window is full. Measured on pi 1.5.2: with both fields `pi --list-models`
+      // shows the window and the request carries `max_tokens: 8192`.
       return {
         content: JSON.stringify(
           {
@@ -336,7 +344,7 @@ function renderConfigFile(
                 apiKey,
                 api: 'openai-completions',
                 authHeader: true,
-                models: [{ id: modelId }],
+                models: [{ id: modelId, ...modelWindowFields(contextLength) }],
               },
             },
           },

test/custom-model-injection.test.ts

@@ -194,6 +194,24 @@ describe('buildCustomModelInjection', () => {
     expect(parsed.providers.custom.models).toEqual([{ id: 'qwen3' }]); // array, NOT keyed by id
   });
 
+  it('pi: the model entry carries the endpoint window and a reply share of it, or pi takes the model for a 128k one', () => {
+    const models = (contextLength?: number) => {
+      const result = buildCustomModelInjection(entryOrThrow('pi'), endpoint, 'qwen3.6:35b-a3b', contextLength);
+      if (result.kind !== 'configDir') throw new Error('unreachable');
+      return JSON.parse(result.files[0].content).providers.custom.models;
+    };
+    // The full window, and 8192 reply tokens at most (measured on pi 1.5.2: the request then
+    // carries max_tokens 8192 instead of its own 16384).
+    expect(models(65536)).toEqual([{ id: 'qwen3.6:35b-a3b', contextWindow: 65536, maxTokens: 8192 }]);
+    expect(models(131072.9)).toEqual([{ id: 'qwen3.6:35b-a3b', contextWindow: 131072, maxTokens: 8192 }]);
+    // A small window keeps three quarters for the prompt.
+    expect(models(16384)).toEqual([{ id: 'qwen3.6:35b-a3b', contextWindow: 16384, maxTokens: 4096 }]);
+    // Unknown stays unknown: no limit is invented.
+    for (const contextLength of [undefined, NaN, 0, -1, Infinity]) {
+      expect(models(contextLength), String(contextLength)).toEqual([{ id: 'qwen3.6:35b-a3b' }]);
+    }
+  });
+
   it('omp: configDir writes .omp/agent/models.yml, redirected via HOME (verified live end-to-end)', () => {
     const result = buildCustomModelInjection(entryOrThrow('omp'), endpoint, 'qwen3');
     if (result.kind !== 'configDir') throw new Error('unreachable');
@@ -239,9 +257,9 @@ describe('buildCustomModelInjection', () => {
     // The patch references the DEDICATED var by name, never dsh's own DEEPSEEK_API_KEY:
     // dsh's llm-deepseek/web-search rows read that too and would leak this key to
     // api.deepseek.com on an in-session /model switch back to the official route.
-    expect(patch.find((row: { id: string }) => row.id === 'llm-pi-ai').config.providers['codeman-custom'].apiKeyEnv).toBe(
-      'CODEMAN_CUSTOM_MODEL_API_KEY'
-    );
+    expect(
+      patch.find((row: { id: string }) => row.id === 'llm-pi-ai').config.providers['codeman-custom'].apiKeyEnv
+    ).toBe('CODEMAN_CUSTOM_MODEL_API_KEY');
     expect(result.files[0].content).not.toContain('DEEPSEEK_API_KEY');
     expect(patch.find((row: { id: string }) => row.id === 'dsh-tui').config.model).toBe('qwen3');
   });