fix(ai): never pass empty messages to AI SDK standardizePrompt (#4102)

pruneMessages({ emptyMessages: 'remove', reasoning: 'all' }) can yield []
when history is reasoning-only after DeepSeek/OpenRouter-style strip.
Guard prepareCattyMessagesForStream, prepareStep, and processCattyStream so
Invalid prompt: messages must not be empty cannot be reached that way.

infrastructure/ai/harness/cattyRuntime.ts

@@ -273,9 +273,18 @@ export function prepareCattyMessagesForStream(
   messages: ModelMessage[],
   options: { preserveReasoning?: boolean } = {},
 ): ModelMessage[] {
-  return pruneMessages({
+  const pruned = pruneMessages({
     messages,
     reasoning: options.preserveReasoning ? 'none' : 'all',
     emptyMessages: 'remove',
   });
+  // pruneMessages({ emptyMessages: 'remove' }) can yield [] when every message
+  // was reasoning-only (or otherwise empty after stripping). AI SDK
+  // standardizePrompt then throws "Invalid prompt: messages must not be empty"
+  // (#4102). Refuse to hand an emptied list downstream when we still had input.
+  if (pruned.length === 0 && messages.length > 0) {
+    console.warn('[Catty] prepareCattyMessagesForStream refused empty prune result (#4102)');
+    return messages;
+  }
+  return pruned;
 }

infrastructure/ai/harness/turnDrivers/cattyMessageBuilder.test.ts

@@ -819,3 +819,55 @@ test('durable summary keeps Anthropic signed thinking for DeepSeek (#4074 P2)',
   assert.ok(reasoning.some((p) => p.text === 'unsigned recent'));
 });
 
+
+test('prepareCattyMessagesForStream does not return [] when prune empties reasoning-only history (#4102)', () => {
+  const reasoningOnly = [
+    {
+      role: 'assistant' as const,
+      content: [{ type: 'reasoning' as const, text: 'plan the ping' }],
+    },
+    {
+      role: 'assistant' as const,
+      content: [{ type: 'reasoning' as const, text: 'then run it' }],
+    },
+  ];
+  const prepared = prepareCattyMessagesForStream(reasoningOnly);
+  assert.ok(prepared.length > 0, 'must not hand AI SDK an empty messages array');
+  assert.equal(prepared.length, reasoningOnly.length);
+  assert.equal(
+    (prepared[0].content as { type: string }[]).some((part) => part.type === 'reasoning'),
+    true,
+    'fallback keeps original reasoning when strip-all would empty the prompt',
+  );
+});
+
+test('prepareCattyMessagesForStream still strips reasoning when other content remains (#4102)', () => {
+  const mixed = [
+    { role: 'user' as const, content: 'ping www.baidu.com -c 4' },
+    {
+      role: 'assistant' as const,
+      content: [
+        { type: 'reasoning' as const, text: 'plan' },
+        { type: 'text' as const, text: 'running it' },
+        {
+          type: 'tool-call' as const,
+          toolCallId: 'call-1',
+          toolName: 'terminal_execute',
+          input: { command: 'ping www.baidu.com -c 4' },
+        },
+      ],
+    },
+  ];
+  const prepared = prepareCattyMessagesForStream(mixed);
+  assert.equal(prepared.length, 2);
+  assert.equal(prepared[0].role, 'user');
+  const assistantContent = prepared[1].content;
+  assert.ok(Array.isArray(assistantContent));
+  assert.equal(assistantContent.some((part) => part.type === 'reasoning'), false);
+  assert.equal(assistantContent.some((part) => part.type === 'text'), true);
+  assert.equal(assistantContent.some((part) => part.type === 'tool-call'), true);
+});
+
+test('prepareCattyMessagesForStream leaves a true empty input empty', () => {
+  assert.deepEqual(prepareCattyMessagesForStream([]), []);
+});

infrastructure/ai/harness/turnDrivers/cattyStreamProcessor.ts

@@ -132,6 +132,9 @@ export async function processCattyStream(input: ProcessCattyStreamInput): Promis
   } = input;
 
   let runtimeContext = initialRuntimeContext;
+  if (!Array.isArray(sdkMessages) || sdkMessages.length === 0) {
+    throw new Error('Invalid prompt: Catty refused an empty messages array (#4102).');
+  }
   const { tools, toolsContext } = toolsBundle;
   const endpoint = input.providerConfig
     ? resolveProviderEndpoint(input.providerConfig, resolveProviderStyle(input.providerConfig), undefined)

infrastructure/ai/harness/turnDrivers/cattyTurnDriver.ts

@@ -518,6 +518,12 @@ async function runCattyTurn(input: CattyTurnInput, ctx: TurnDriverContext): Prom
               stepMessages = stripped.messages;
             }
           }
+          // Never replace the step prompt with [] (#4102). Prefer the SDK's
+          // pre-prepare messages over an emptied prepareStepContext result.
+          if (stepMessages.length === 0 && messages.length > 0) {
+            console.warn('[Catty] prepareStep produced empty messages; keeping prior step messages (#4102)');
+            stepMessages = messages;
+          }
           emitContextSnapshot(stepMessages);
           return {
             messages: stepMessages,