feat(agent): Comprehensive reasoning token support across providers

Added provider-specific reasoning/thinking token support for:
- OpenAI (o1, o3, gpt-5): Full reasoning events via Responses API, token counts via Chat Completions
- Groq: reasoning_format:"parsed" for Chat Completions, no summary support for Responses
- Gemini 2.5: extra_body.google.thinking_config with <thought> tag extraction
- OpenRouter: Unified reasoning parameter with message.reasoning field
- Anthropic: Limited support via OpenAI compatibility layer

Key improvements:
- Centralized provider detection based on baseURL
- parseReasoningFromMessage() extracts provider-specific reasoning content
- adjustRequestForProvider() handles provider-specific request modifications
- Smart reasoning support detection with caching per API type
- Comprehensive README documentation with provider support matrix

Fixes reasoning tokens not appearing for GPT-5 and other reasoning models.

packages/agent/README.md

@@ -33,14 +33,20 @@ pi-agent
 # Continue most recently modified session in current directory
 pi-agent --continue "Follow up question"
 
-# GPT-OSS via Groq
+# GPT-OSS via Groq (supports reasoning with both APIs)
 pi-agent --base-url https://api.groq.com/openai/v1 --api-key $GROQ_API_KEY --model openai/gpt-oss-120b
 
 # GLM 4.5 via OpenRouter
 pi-agent --base-url https://openrouter.ai/api/v1 --api-key $OPENROUTER_API_KEY --model z-ai/glm-4.5
 
-# Claude via Anthropic (no prompt caching support - see https://docs.anthropic.com/en/api/openai-sdk)
+# Claude via Anthropic's OpenAI compatibility layer
+# Note: No prompt caching or thinking content support. For full features, use the native Anthropic API.
+# See: https://docs.anthropic.com/en/api/openai-sdk
 pi-agent --base-url https://api.anthropic.com/v1 --api-key $ANTHROPIC_API_KEY --model claude-opus-4-1-20250805
+
+# Gemini via Google AI (set GEMINI_API_KEY environment variable)
+# Note: Gemini 2.5 models support reasoning but require extra_body for thinking content (not yet supported)
+pi-agent --base-url https://generativelanguage.googleapis.com/v1beta/openai/ --api-key $GEMINI_API_KEY --model gemini-2.5-flash
 ```
 
 ## Usage Modes
@@ -137,13 +143,18 @@ When using `--json`, the agent outputs these event types:
 - `user_message` - User input
 - `assistant_start` - Assistant begins responding
 - `assistant_message` - Assistant's response
-- `thinking` - Reasoning/thinking (for models that support it)
+- `thinking` - Reasoning/thinking (for models that support it, requires `--api responses`)
 - `tool_call` - Tool being called
 - `tool_result` - Result from tool
-- `token_usage` - Token usage statistics
+- `token_usage` - Token usage statistics (includes `reasoningTokens` for models with reasoning)
 - `error` - Error occurred
 - `interrupted` - Processing was interrupted
 
+**Note:**
+- OpenAI's Chat Completions API (`--api completions`, the default) only returns reasoning token *counts* but not the actual thinking content. To see thinking events, use the Responses API with `--api responses` for supported models (o1, o3, gpt-5).
+- Anthropic's OpenAI compatibility layer doesn't return thinking content. Use the native Anthropic API for full extended thinking features.
+- Gemini 2.5 models support reasoning via `reasoning_effort` but require the `extra_body` parameter with `thinking_config.include_thoughts: true` to get thinking content, which is not yet supported in pi-agent.
+
 The complete TypeScript type definition for `AgentEvent` can be found in [`src/agent.ts`](src/agent.ts#L6).
 
 ## Build an Interactive UI with JSON Mode
@@ -284,6 +295,66 @@ agent.on('error', (err) => {
 console.log('Pi Agent Interactive Chat');
 ```
 
+## Reasoning
+
+Pi-agent supports reasoning/thinking tokens for models that provide this capability:
+
+### Supported Providers
+
+| Provider | API | Reasoning Tokens | Thinking Content | Notes |
+|----------|-----|------------------|------------------|-------|
+| OpenAI (o1, o3) | Responses | ✅ | ✅ | Full support via `reasoning` events |
+| OpenAI (o1, o3) | Chat Completions | ✅ | ❌ | Token counts only, no content |
+| OpenAI (gpt-5) | Responses | ✅ | ⚠️ | Model returns empty summaries |
+| OpenAI (gpt-5) | Chat Completions | ✅ | ❌ | Token counts only |
+| Groq (gpt-oss) | Responses | ✅ | ❌ | No reasoning.summary support |
+| Groq (gpt-oss) | Chat Completions | ✅ | ✅ | Via `reasoning_format: "parsed"` |
+| Gemini 2.5 | Chat Completions | ✅ | ✅ | Via `extra_body.google.thinking_config` |
+| Anthropic | OpenAI Compat | ❌ | ❌ | Not supported in compatibility layer |
+| OpenRouter | Various | ✅ | ✅ | Model-dependent, see provider docs |
+
+### Usage Examples
+
+```bash
+# OpenAI o1/o3 - see thinking content with Responses API
+pi-agent --api responses --model o1-mini "Explain quantum computing"
+
+# Groq gpt-oss - reasoning with Chat Completions
+pi-agent --base-url https://api.groq.com/openai/v1 --api-key $GROQ_API_KEY \
+  --model openai/gpt-oss-120b "Complex math problem"
+
+# Gemini 2.5 - thinking content automatically configured
+pi-agent --base-url https://generativelanguage.googleapis.com/v1beta/openai/ \
+  --api-key $GEMINI_API_KEY --model gemini-2.5-flash "Think step by step"
+
+# OpenRouter - supports various reasoning models
+pi-agent --base-url https://openrouter.ai/api/v1 --api-key $OPENROUTER_API_KEY \
+  --model "qwen/qwen3-235b-a22b-thinking-2507" "Complex reasoning task"
+```
+
+### JSON Mode Events
+
+When reasoning is active, you'll see:
+- `reasoning` events with thinking text (when available)
+- `token_usage` events include `reasoningTokens` field
+- Console/TUI show reasoning tokens with ⚡ symbol
+
+### Technical Details
+
+The agent automatically:
+- Detects provider from base URL
+- Tests model reasoning support on first use (cached)
+- Adjusts request parameters per provider:
+  - OpenAI: `reasoning_effort` (minimal/low)
+  - Groq: `reasoning_format: "parsed"`
+  - Gemini: `extra_body.google.thinking_config`
+  - OpenRouter: `reasoning` object with `effort` field
+- Parses provider-specific response formats:
+  - Gemini: Extracts from `<thought>` tags
+  - Groq: Uses `message.reasoning` field
+  - OpenRouter: Uses `message.reasoning` field
+  - OpenAI: Uses standard `reasoning` events
+
 ## Architecture
 
 The agent is built with: