fix(engine): teach the router with examples and pin its sampling

engine/.env

@@ -12,7 +12,7 @@ STIRLING_FAST_MODEL=anthropic:claude-haiku-4-5
 
 # Default output token limits applied by the engine for each model tier.
 STIRLING_SMART_MODEL_MAX_TOKENS=8192
-STIRLING_FAST_MODEL_MAX_TOKENS=2048
+STIRLING_FAST_MODEL_MAX_TOKENS=4096
 
 # Process-wide cap on concurrent model API calls, shared by both model tiers.
 # Per-request fan-outs (chunked reasoner workers, contradiction detection) are

engine/src/stirling/agents/orchestrator.py

@@ -7,6 +7,7 @@ from typing import Literal, assert_never
 from pydantic import ConfigDict, Field
 from pydantic_ai import Agent
 from pydantic_ai.output import NativeOutput, ToolOutput
+from pydantic_ai.settings import ModelSettings
 from pydantic_ai.tools import RunContext
 
 from stirling.agents.output_mode import output_retries, uses_tool_output
@@ -65,10 +66,34 @@ _ROUTER_SYSTEM_PROMPT = (
     "- pdf_create: generate a NEW document from scratch (invoice, report, letter) - no input file.\n"
     "- unsupported: none of the above fit, or the user asks about the assistant itself; put a "
     "helpful message in 'message'.\n"
-    "Respond with the capability and (only for unsupported) a message."
+    "Respond with the capability and (only for unsupported) a message.\n"
+    "\n"
+    "Decide by what the user wants BACK, not by which PDF words appear in the message. "
+    "Wanting an answer, or information read out of the document, is pdf_question. Wanting a "
+    "changed file handed back is pdf_edit. A document being mentioned is not a request to act "
+    "on it.\n"
+    "\n"
+    "Examples:\n"
+    '  "How long is the notice period?" -> pdf_question\n'
+    '  "Shorten the notice period to 30 days" -> pdf_edit\n'
+    '  "Is there a table of contents?" -> pdf_question\n'
+    '  "Add a table of contents" -> pdf_edit\n'
+    '  "Does it say anything about encryption?" -> pdf_question\n'
+    '  "Encrypt this file" -> pdf_edit\n'
+    '  "What sections cover the merger?" -> pdf_question\n'
+    '  "Combine these into one file" -> pdf_edit\n'
+    '  "Tell me if the numbers add up" -> pdf_question\n'
+    '  "Put a note on every paragraph that needs work" -> pdf_review\n'
+    '  "Build me a purchase order from scratch" -> pdf_create\n'
+    '  "Make a rule that stamps every upload" -> user_spec\n'
+    '  "Which version of the app is this?" -> unsupported'
 )
 
 
+def _router_model_settings(runtime: AppRuntime) -> ModelSettings:
+    return {**runtime.fast_model_settings, "temperature": 0.0}
+
+
 class OrchestratorAgent:
     def __init__(self, runtime: AppRuntime) -> None:
         self.runtime = runtime
@@ -146,7 +171,7 @@ class OrchestratorAgent:
                 output_type=NativeOutput([_RouteDecision]),
                 retries=output_retries(runtime.settings.chat_provider),
                 system_prompt=_ROUTER_SYSTEM_PROMPT,
-                model_settings=runtime.fast_model_settings,
+                model_settings=_router_model_settings(runtime),
             )
             if self._route_via_enum
             else None