fix: always send maxOutputTokens; default 16000 (#915)

Unset does not mean the model's maximum — the provider fills in its own, and
Bedrock's is 4096 (measured: converse with no inferenceConfig on
us.anthropic.claude-opus-5 returns stopReason=max_tokens at exactly 4096).

A 30-cell diagram is ~3000 tokens of XML, so anything larger arrived as truncated
JSON and nothing reached the canvas. Small diagrams fitted, which made it look
intermittent.

app/api/chat/route.ts

@@ -493,9 +493,9 @@ IMPORTANT: The "Current diagram XML" is the SINGLE SOURCE OF TRUTH for what's on
     const result = streamText({
         model,
         abortSignal: req.signal,
-        ...(process.env.MAX_OUTPUT_TOKENS && {
-            maxOutputTokens: parseInt(process.env.MAX_OUTPUT_TOKENS, 10),
-        }),
+        // Must be sent: unset means the provider's own default, and Bedrock's is 4096 —
+        // enough for a small diagram, so larger ones were cut off mid-attribute.
+        maxOutputTokens: Number(process.env.MAX_OUTPUT_TOKENS) || 16000,
         stopWhen: stepCountIs(5),
         // Repair truncated tool calls when maxOutputTokens is reached mid-JSON
         experimental_repairToolCall: async ({ toolCall, error }) => {

env.example

@@ -11,6 +11,9 @@ AI_PROVIDER=bedrock
 # Example: AI_MODEL=doubao-seed-1-8-251215,doubao-seed-1-6-flash,doubao-seed-1-6-pro
 AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 
+# Output limit, all providers (default: 16000). Raise it if large diagrams arrive cut off.
+# MAX_OUTPUT_TOKENS=16000
+
 # AWS Bedrock Configuration
 # AWS_REGION=us-east-1
 # AWS_ACCESS_KEY_ID=your-access-key-id