fix(ai-mistral): report model_length stop as a length finish

.changeset/mistral-model-length-finish-reason.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/ai-mistral': patch
+---
+
+A Mistral response that stops with `finish_reason` `model_length` (the model's context window is exhausted) now reports `finishReason: 'length'` on `RUN_FINISHED` and `structuredOutput` reports it as a context-window truncation (shorten the prompt or use a larger-context model, not raise `max_tokens`). Before, a streamed `model_length` stop was reported as a normal `stop`, and `structuredOutput` surfaced a JSON parse error instead of the truncation error.

packages/ai-mistral/src/adapters/text.ts

@@ -497,7 +497,16 @@ export class MistralTextAdapter<
     // A response cut off at the output cap is a truncated JSON document;
     // report it before the parse error (issue #1426). The errors carry the
     // usage, because the provider already billed the tokens.
-    if (response.choices[0]?.finishReason === 'length') {
+    const structuredFinishReason = response.choices[0]?.finishReason
+    if (structuredFinishReason === 'model_length') {
+      throw Object.assign(
+        new Error(
+          'mistral.structuredOutput: the response was cut off because the model reached the end of its context window (finish_reason=model_length); shorten the prompt or use a model with a larger context window',
+        ),
+        usage && { usage },
+      )
+    }
+    if (structuredFinishReason === 'length') {
       throw Object.assign(
         new Error(
           'mistral.structuredOutput: the response was cut off because the maximum token limit was reached (finish_reason=length); raise modelOptions.max_tokens',
@@ -813,7 +822,7 @@ export class MistralTextAdapter<
           const computedFinishReason =
             finishReason === 'tool_calls' || hasEmittedToolCall
               ? 'tool_calls'
-              : finishReason === 'length'
+              : finishReason === 'length' || finishReason === 'model_length'
                 ? 'length'
                 : 'stop'
 

packages/ai-mistral/tests/mistral-adapter.test.ts

@@ -228,6 +228,28 @@ describe('Mistral adapters', () => {
       )
     })
 
+    it("reports a 'model_length' stop as truncation in structured output", async () => {
+      mockComplete.mockReset().mockResolvedValue({
+        choices: [
+          { message: { content: '{"value":' }, finishReason: 'model_length' },
+        ],
+      })
+      const adapter = createMistralText('mistral-large-latest', 'test-api-key')
+
+      await expect(
+        adapter.structuredOutput({
+          chatOptions: chatOpts({
+            model: 'mistral-large-latest',
+            messages: [{ role: 'user', content: 'Return structured output' }],
+          }),
+          outputSchema: {
+            type: 'object',
+            properties: { value: { type: 'string' } },
+          },
+        }),
+      ).rejects.toThrow(/end of its context window.*finish_reason=model_length/)
+    })
+
     it('reports a length stop as truncation in structured output', async () => {
       mockComplete.mockReset().mockResolvedValue({
         choices: [
@@ -476,6 +498,37 @@ describe('Mistral AG-UI event emission', () => {
     }
   })
 
+  it("reports Mistral's 'model_length' stop as a length finish", async () => {
+    setupMockStream([
+      {
+        id: 'cmpl-123',
+        model: 'mistral-large-latest',
+        choices: [
+          { index: 0, delta: { content: 'Hello' }, finishReason: null },
+        ],
+      },
+      {
+        id: 'cmpl-123',
+        model: 'mistral-large-latest',
+        choices: [{ index: 0, delta: {}, finishReason: 'model_length' }],
+      },
+    ])
+    const adapter = createMistralText('mistral-large-latest', 'test-api-key')
+    const chunks: Array<AdapterYieldChunk> = []
+
+    for await (const chunk of adapter.chatStream(
+      chatOpts({
+        model: 'mistral-large-latest',
+        messages: [{ role: 'user', content: 'Hello' }],
+      }),
+    )) {
+      chunks.push(chunk)
+    }
+
+    const runFinishedChunk = chunks.find((c) => c.type === 'RUN_FINISHED')
+    expect(runFinishedChunk).toMatchObject({ finishReason: 'length' })
+  })
+
   it('emits AG-UI tool call events', async () => {
     const streamChunks = [
       {