fix(ai): support OpenRouter session affinity (#6496)

Fixes #6366

packages/ai/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Breaking Changes
+
+- Removed the `OpenAIResponsesCompat.sendSessionIdHeader` flag. Session-affinity behavior is now controlled by `compat.sessionAffinityFormat` (`"openai"`, `"openai-nosession"`, or `"openrouter"`). Replace `sendSessionIdHeader: false` with `sessionAffinityFormat: "openai-nosession"` ([#6366](https://github.com/earendil-works/pi/issues/6366)).
+
 ### Added
 
 - Added cache-friendly dynamic tool loading. `ToolResultMessage.addedToolNames` marks where tools from `Context.tools` became available; Anthropic and OpenAI Responses use native deferred loading so late tools stay out of the cached prefix, while other providers continue using `Context.tools` normally ([#6474](https://github.com/earendil-works/pi-mono/pull/6474)).
@@ -14,6 +18,7 @@
 - Fixed OpenRouter model context windows to use the top provider's actual context length ([#6481](https://github.com/earendil-works/pi-mono/pull/6481) by [@davidbrai](https://github.com/davidbrai)).
 - Fixed the GitHub Copilot `mai-code-1-flash-picker` model to route through the `/responses` endpoint.
 - Fixed Amazon Bedrock requests to use the generic `apiKey` stream option as a Bedrock bearer token.
+- Fixed OpenRouter OpenAI-compatible session IDs to use the `x-session-id` header instead of OpenAI-specific session-affinity fields ([#6366](https://github.com/earendil-works/pi/issues/6366)).
 
 ## [0.80.6] - 2026-07-09