feat(providers): default ChatGPT OAuth to GPT-5.5

Default ChatGPT Subscription (OAuth) provider model selection to GPT-5.5 and update model metadata, tests, and docs.

docs/02-providers.md

@@ -62,7 +62,7 @@ Streaming fallback is conservative: backup models are tried only if the stream f
 |----------|------|----------|---------------|
 | **anthropic** | Native HTTP + SSE | API key required | `claude-sonnet-4-5-20250929` |
 | **claude_cli** | stdio subprocess + MCP | Binary path (default: `claude`) | `sonnet` |
-| **codex** | OAuth Responses API | OAuth token source | `gpt-5.3-codex` |
+| **codex** | OAuth Responses API | OAuth token source | `gpt-5.5` |
 | **acp** | JSON-RPC 2.0 subagents | Binary + workspace dir | `claude` |
 | **dashscope** | OpenAI-compat wrapper | API key + custom models | `qwen3-max` |
 | **openai** (+ 10+ variants) | OpenAI-compatible | API key + endpoint URL | Model-specific |
@@ -571,7 +571,7 @@ Claude CLI inherits thinking support from the underlying Claude model. Thinking
 
 ## 12. Codex Provider
 
-The Codex provider integrates with OpenAI's ChatGPT Responses API (OAuth-based), enabling access to gpt-5.3-codex model through the chatgpt.com backend. Unlike standard OpenAI endpoints, Codex uses OAuth token refresh and a custom response format with "phase" markers.
+The Codex provider integrates with OpenAI's ChatGPT Responses API (OAuth-based), defaulting to `gpt-5.5` through the chatgpt.com backend. Unlike standard OpenAI endpoints, Codex uses OAuth token refresh and a custom response format with "phase" markers.
 
 ### Configuration
 
@@ -582,7 +582,7 @@ tokenSource := &MyTokenSource{} // implements TokenSource interface
 provider := NewCodexProvider("codex", tokenSource, "", "")
 // or specify custom API base and model:
 provider := NewCodexProvider("codex", tokenSource,
-  "https://chatgpt.com/backend-api", "gpt-5.3-codex")
+  "https://chatgpt.com/backend-api", "gpt-5.5")
 ```
 
 ### API Endpoint
@@ -601,7 +601,7 @@ Codex returns structured responses with phase markers:
 ```json
 {
   "id": "...",
-  "model": "gpt-5.3-codex",
+  "model": "gpt-5.5",
   "choices": [{
     "message": {
       "role": "assistant",

docs/18-http-api.md

@@ -148,7 +148,7 @@ can override or inherit routing behavior while keeping the main `provider` field
 ```json
 {
   "provider": "openai-codex",
-  "model": "gpt-5.4",
+  "model": "gpt-5.5",
   "chatgpt_oauth_routing": {
     "override_mode": "custom",
     "strategy": "round_robin"
@@ -214,7 +214,7 @@ Agents can now store capability-aware GPT-5/Codex reasoning intent under top-lev
 ```json
 {
   "provider": "openai-codex",
-  "model": "gpt-5.4",
+  "model": "gpt-5.5",
   "reasoning_config": {
     "override_mode": "inherit"
   }
@@ -364,11 +364,11 @@ Example response:
 {
   "models": [
     {
-      "id": "gpt-5.4",
-      "name": "GPT-5.4",
+      "id": "gpt-5.5",
+      "name": "GPT-5.5",
       "reasoning": {
         "levels": ["none", "low", "medium", "high", "xhigh"],
-        "default_effort": "none"
+        "default_effort": "medium"
       }
     },
     {

docs/project-changelog.md

@@ -6,6 +6,18 @@ Significant changes, features, and fixes in reverse chronological order.
 
 ## 2026-05-18
 
+### Providers: ChatGPT OAuth GPT-5.5 default
+
+**Changed**
+
+- Updated ChatGPT Subscription (OAuth) default model, provider model catalog, reasoning metadata, and docs examples to prefer `gpt-5.5`.
+
+**Tests**
+
+- Updated focused provider/model catalog, reasoning, registry, and token-window coverage for `gpt-5.5`.
+
+---
+
 ### Deployment: Codex CLI service-user auth
 
 **Fixes**