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+# Anthropic Thinking Support — Implementation Plan
+
+## Overview
+
+Add `Thinking`, `Effort`, `thinking_budget_tokens`, and `thinking_interleaved`
+fields to `AnthropicMessagesConfig` and wire them through to the Anthropic
+Messages API. OpenAI config is unchanged.
+
+## Design decisions
+
+- `Thinking = .enabled` by default (compatible with all current models incl. Haiku 4.5)
+- `Effort = .medium` by default (only used when `thinking = .adaptive`)
+- `thinking_budget_tokens: ?u32 = 32_000` (quality/cost shoulder per Anthropic docs; ignored when adaptive)
+- `thinking_interleaved: bool = false` (requires explicit opt-in; beta header 400s on unsupported models)
+- Always send `display: "summarized"` to get visible thinking text in all modes
+- `effort` is only emitted when `thinking == .adaptive`
+- `budget_tokens` uses `thinking_budget_tokens orelse (max_tokens - 1)` when `.enabled` without interleaving
+- `interleaved-thinking-2025-05-14` beta header only sent when `thinking == .enabled && thinking_interleaved == true`
+- `WireIdentity` for Anthropic extended with thinking/effort/budget/interleaved so sessions with different thinking configs are distinct
+
+---
+
+## Phase 1 — Config types and `AnthropicMessagesConfig` fields ✅ COMPLETE
+
+**Files:** `libpanto/src/config.zig`
+
+- Add `Thinking` enum: `disabled`, `enabled`, `adaptive`
+- Add `Effort` enum: `low`, `medium`, `high`, `xhigh`, `max`
+- Add fields to `AnthropicMessagesConfig`:
+  - `thinking: Thinking = .enabled`
+  - `effort: Effort = .medium`
+  - `thinking_budget_tokens: ?u32 = 32_000`
+  - `thinking_interleaved: bool = false`
+- Update `WireIdentity` to include `thinking`, `effort`, `thinking_budget_tokens`,
+  `thinking_interleaved` for Anthropic; `wireIdentity()` on Anthropic variant fills them in
+- Update tests in `config.zig`
+
+---
+
+## Phase 2 — Wire serialization (`anthropic_messages_json.zig`) ✅ COMPLETE
+
+**Files:** `libpanto/src/anthropic_messages_json.zig`
+
+- In `serializeRequest`, after the system prompt block:
+  - `thinking == .disabled` → nothing
+  - `thinking == .enabled` → emit `thinking: { type: "enabled", budget_tokens: B, display: "summarized" }`
+    where `B = thinking_budget_tokens orelse (max_tokens - 1)`, clamped so `B < max_tokens`
+  - `thinking == .adaptive` → emit `thinking: { type: "adaptive", display: "summarized" }` + top-level `effort: "<level>"`
+- Add unit tests covering all three modes, budget fallback/clamp, effort gating
+
+---
+
+## Phase 3 — Beta header in provider (`provider_anthropic_messages.zig`) ✅ COMPLETE
+
+**Files:** `libpanto/src/provider_anthropic_messages.zig`
+
+- In `AnthropicMessagesRequest.open`, add `anthropic-beta` header only when
+  `config.thinking == .enabled && config.thinking_interleaved == true`
+- Value: `"interleaved-thinking-2025-05-14"`
+- Add unit test (or update existing open/serialize path test) confirming header presence/absence
+
+---
+
+## Phase 4 — Session persistence ✅ COMPLETE
+
+**Files:** `libpanto/src/session.zig`, `libpanto/src/session_store.zig`,
+`libpanto/src/file_system_jsonl_store.zig`, `libpanto/src/null_store.zig`
+
+- Extend the stamp serialization / deserialization in `session.zig` to
+  read/write `thinking`, `effort`, `thinking_budget_tokens`, `thinking_interleaved`
+  from/to JSONL
+- Update `WireIdentity` in `session_store.zig` (distinct from config's) to carry the same fields
+- Update `null_store.zig` and `file_system_jsonl_store.zig` default stamps
+- Add round-trip tests
+
+---
+
+## Phase 5 — `models.toml` parsing and `buildProviderConfig` ✅ COMPLETE
+
+**Files:** `src/models_toml.zig`, `src/config_file.zig`
+
+- Add Anthropic fields to `ModelDef`: `thinking`, `effort`, `thinking_budget_tokens`, `thinking_interleaved`
+- Hand-parse all four from TOML in `ingestModel` (string→enum for thinking/effort, int for budget, bool for interleaved)
+- Thread them through `buildProviderConfig` into `AnthropicMessagesConfig`
+- Update schema doc comment in `models_toml.zig` and sample config in `src/luarocks_runtime.zig`
+- Add parse and buildProviderConfig tests
+
+---
+
+## Phase 6 — Build verification and test run ✅ COMPLETE
+
+- `zig build` clean
+- `zig build test` passes (all existing + new tests)
+- Smoke-check that the TOML sample in `luarocks_runtime.zig` is valid
+
+---