anthropic thinking

6 files changed, 574 insertions, 8 deletions

diff --git a/libpanto/src/anthropic_messages_json.zig b/libpanto/src/anthropic_messages_json.zig
index bccf692..78f4b5f 100644
--- a/libpanto/src/anthropic_messages_json.zig
+++ b/libpanto/src/anthropic_messages_json.zig
@@ -52,6 +52,41 @@ pub fn serializeRequest(
     try s.objectField("stream");
     try s.write(true);
 
+    // Extended thinking configuration.
+    // `.disabled`  — omit the field entirely.
+    // `.enabled`   — manual budget: { type, budget_tokens, display }.
+    // `.adaptive`  — adaptive mode: { type, display } + top-level effort.
+    switch (cfg.thinking) {
+        .disabled => {},
+        .enabled => {
+            // Resolve budget: explicit value or fall back to max_tokens - 1.
+            // Clamp so budget is always strictly less than max_tokens (required
+            // by Anthropic when not using interleaved thinking).
+            const raw_budget: u32 = cfg.thinking_budget_tokens orelse (cfg.max_tokens -| 1);
+            const budget: u32 = if (raw_budget >= cfg.max_tokens) cfg.max_tokens -| 1 else raw_budget;
+            try s.objectField("thinking");
+            try s.beginObject();
+            try s.objectField("type");
+            try s.write("enabled");
+            try s.objectField("budget_tokens");
+            try s.write(budget);
+            try s.objectField("display");
+            try s.write("summarized");
+            try s.endObject();
+        },
+        .adaptive => {
+            try s.objectField("thinking");
+            try s.beginObject();
+            try s.objectField("type");
+            try s.write("adaptive");
+            try s.objectField("display");
+            try s.write("summarized");
+            try s.endObject();
+            try s.objectField("effort");
+            try s.write(@tagName(cfg.effort));
+        },
+    }
+
     // Build and emit the concatenated system prompt, if any.
     var system_buf: std.ArrayList(u8) = .empty;
     defer system_buf.deinit(allocator);
@@ -1134,6 +1169,163 @@ test "serializeRequest - tool result with image part emits image block" {
     try testing.expectEqualStrings("iVBOR==", src.get("data").?.string);
 }
 
+test "serializeRequest - thinking disabled omits thinking and effort fields" {
+    const allocator = testing.allocator;
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+    try conv.addUserMessage("hi");
+
+    var cfg = testConfig("claude-x");
+    cfg.thinking = .disabled;
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    const root = parsed.value.object;
+    try testing.expect(root.get("thinking") == null);
+    try testing.expect(root.get("effort") == null);
+}
+
+test "serializeRequest - thinking enabled explicit budget" {
+    const allocator = testing.allocator;
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+    try conv.addUserMessage("hi");
+
+    var cfg = testConfig("claude-x"); // max_tokens = 1024
+    cfg.thinking = .enabled;
+    cfg.thinking_budget_tokens = 500; // within max_tokens
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    const root = parsed.value.object;
+    const th = root.get("thinking").?.object;
+    try testing.expectEqualStrings("enabled", th.get("type").?.string);
+    try testing.expectEqual(@as(i64, 500), th.get("budget_tokens").?.integer);
+    try testing.expectEqualStrings("summarized", th.get("display").?.string);
+    try testing.expect(root.get("effort") == null);
+}
+
+test "serializeRequest - thinking enabled null budget falls back to max_tokens - 1" {
+    const allocator = testing.allocator;
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+    try conv.addUserMessage("hi");
+
+    var cfg = testConfig("claude-x"); // max_tokens = 1024
+    cfg.thinking = .enabled;
+    cfg.thinking_budget_tokens = null;
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    const th = parsed.value.object.get("thinking").?.object;
+    try testing.expectEqual(@as(i64, 1023), th.get("budget_tokens").?.integer);
+}
+
+test "serializeRequest - thinking enabled budget clamped when >= max_tokens" {
+    const allocator = testing.allocator;
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+    try conv.addUserMessage("hi");
+
+    var cfg = testConfig("claude-x"); // max_tokens = 1024
+    cfg.thinking = .enabled;
+    cfg.thinking_budget_tokens = 2_000; // > max_tokens
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    const th = parsed.value.object.get("thinking").?.object;
+    // Clamped to max_tokens - 1 = 1023
+    try testing.expectEqual(@as(i64, 1023), th.get("budget_tokens").?.integer);
+}
+
+test "serializeRequest - thinking adaptive default effort" {
+    const allocator = testing.allocator;
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+    try conv.addUserMessage("hi");
+
+    var cfg = testConfig("claude-x");
+    cfg.thinking = .adaptive;
+    // effort defaults to .medium
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    const root = parsed.value.object;
+    const th = root.get("thinking").?.object;
+    try testing.expectEqualStrings("adaptive", th.get("type").?.string);
+    try testing.expectEqualStrings("summarized", th.get("display").?.string);
+    try testing.expect(th.get("budget_tokens") == null);
+    try testing.expectEqualStrings("medium", root.get("effort").?.string);
+}
+
+test "serializeRequest - thinking adaptive explicit effort levels" {
+    const allocator = testing.allocator;
+    const efforts = [_]struct { e: config_mod.Effort, name: []const u8 }{
+        .{ .e = .low, .name = "low" },
+        .{ .e = .medium, .name = "medium" },
+        .{ .e = .high, .name = "high" },
+        .{ .e = .xhigh, .name = "xhigh" },
+        .{ .e = .max, .name = "max" },
+    };
+    for (efforts) |entry| {
+        var conv = conversation.Conversation.init(allocator);
+        defer conv.deinit();
+        try conv.addUserMessage("hi");
+
+        var cfg = testConfig("claude-x");
+        cfg.thinking = .adaptive;
+        cfg.effort = entry.e;
+        var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+        defer empty_tools.deinit();
+        const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+        defer allocator.free(body);
+
+        var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+        defer parsed.deinit();
+        try testing.expectEqualStrings(entry.name, parsed.value.object.get("effort").?.string);
+    }
+}
+
+test "serializeRequest - thinking adaptive ignores budget_tokens" {
+    const allocator = testing.allocator;
+    var conv = conversation.Conversation.init(allocator);
+    defer conv.deinit();
+    try conv.addUserMessage("hi");
+
+    var cfg = testConfig("claude-x");
+    cfg.thinking = .adaptive;
+    cfg.thinking_budget_tokens = 9_999; // should be ignored
+    var empty_tools = tool_registry_mod.ToolRegistry.init(allocator);
+    defer empty_tools.deinit();
+    const body = try serializeRequest(allocator, &cfg, &conv, &empty_tools);
+    defer allocator.free(body);
+
+    var parsed = try std.json.parseFromSlice(std.json.Value, allocator, body, .{});
+    defer parsed.deinit();
+    const th = parsed.value.object.get("thinking").?.object;
+    try testing.expect(th.get("budget_tokens") == null);
+}
+
 test "serializeRequest - empty ToolUse input becomes {} not invalid JSON" {
     const allocator = testing.allocator;
 
diff --git a/libpanto/src/config.zig b/libpanto/src/config.zig
index 5e01bb1..587f00a 100644
--- a/libpanto/src/config.zig
+++ b/libpanto/src/config.zig
@@ -41,6 +41,28 @@ pub const ReasoningEffort = enum {
     high,
 };
 
+/// Anthropic extended-thinking mode.
+pub const Thinking = enum {
+    /// Do not request extended thinking.
+    disabled,
+    /// Manual extended thinking with an explicit token budget (`thinking_budget_tokens`).
+    /// Supported on all current models including Haiku 4.5. Default.
+    enabled,
+    /// Adaptive thinking: Claude decides when and how much to think.
+    /// Requires Opus 4.6 / Sonnet 4.6 or newer; automatic on Opus 4.7+.
+    adaptive,
+};
+
+/// Effort level sent with Anthropic adaptive thinking (`thinking = .adaptive`).
+/// Ignored when `thinking` is `.enabled` or `.disabled`.
+pub const Effort = enum {
+    low,
+    medium,
+    high,
+    xhigh,
+    max,
+};
+
 pub const OpenAIChatConfig = struct {
     api_key: []const u8,
     base_url: []const u8,
@@ -57,6 +79,22 @@ pub const AnthropicMessagesConfig = struct {
     api_version: []const u8 = "2023-06-01",
     /// Required by Anthropic's Messages API.
     max_tokens: u32 = 64_000,
+    /// Extended-thinking mode. `.enabled` sends a manual thinking block with
+    /// `thinking_budget_tokens`; `.adaptive` lets Claude decide and uses
+    /// `effort` instead. `.disabled` omits thinking entirely.
+    thinking: Thinking = .enabled,
+    /// Effort level for adaptive thinking. Only emitted on the wire when
+    /// `thinking == .adaptive`; ignored otherwise.
+    effort: Effort = .medium,
+    /// Maximum tokens Claude may spend on internal reasoning when
+    /// `thinking == .enabled`. `null` falls back to `max_tokens - 1`.
+    /// Ignored when `thinking == .adaptive` or `.disabled`.
+    thinking_budget_tokens: ?u32 = 32_000,
+    /// When true and `thinking == .enabled`, sends the
+    /// `interleaved-thinking-2025-05-14` beta header so Claude can think
+    /// between tool calls. Ignored when `thinking == .adaptive` (interleaving
+    /// is automatic there) or `.disabled`.
+    thinking_interleaved: bool = false,
 };
 
 /// Per-provider transport/auth/model configuration. Tagged by `APIStyle`.
@@ -70,8 +108,8 @@ pub const ProviderConfig = union(APIStyle) {
 
     /// The wire-format provider identity for this config: the ground-truth
     /// `{api_style, base_url, model, reasoning}` that a turn is sent with.
-    /// Anthropic has no reasoning-effort knob, so it reports `.default`.
-    /// Borrowed slices; valid as long as the config is.
+    /// Anthropic carries thinking/effort/budget/interleaved instead of
+    /// reasoning. Borrowed slices; valid as long as the config is.
     pub fn wireIdentity(self: ProviderConfig) WireIdentity {
         return switch (self) {
             .openai_chat => |c| .{
@@ -84,7 +122,10 @@ pub const ProviderConfig = union(APIStyle) {
                 .api_style = .anthropic_messages,
                 .base_url = c.base_url,
                 .model = c.model,
-                .reasoning = .default,
+                .thinking = c.thinking,
+                .effort = c.effort,
+                .thinking_budget_tokens = c.thinking_budget_tokens,
+                .thinking_interleaved = c.thinking_interleaved,
             },
         };
     }
@@ -93,11 +134,24 @@ pub const ProviderConfig = union(APIStyle) {
 /// Wire-format provider identity (see `ProviderConfig.wireIdentity`). This
 /// is the same shape as `session_store.WireIdentity`; defined here to avoid
 /// a module cycle (config must not import session_store).
+///
+/// OpenAI uses `reasoning`; Anthropic uses `thinking`/`effort`/
+/// `thinking_budget_tokens`/`thinking_interleaved`. Fields unused by the
+/// active provider carry their zero/default values.
 pub const WireIdentity = struct {
     api_style: APIStyle,
     base_url: []const u8,
     model: []const u8,
+    /// OpenAI only.
     reasoning: ReasoningEffort = .default,
+    /// Anthropic only.
+    thinking: Thinking = .enabled,
+    /// Anthropic only; only meaningful when `thinking == .adaptive`.
+    effort: Effort = .medium,
+    /// Anthropic only; only meaningful when `thinking == .enabled`.
+    thinking_budget_tokens: ?u32 = 32_000,
+    /// Anthropic only; only meaningful when `thinking == .enabled`.
+    thinking_interleaved: bool = false,
 };
 
 /// Compaction settings the agent consults when summarizing old history.
@@ -220,6 +274,30 @@ test "ProviderConfig - anthropic_messages variant" {
     try t.expectEqual(APIStyle.anthropic_messages, cfg.style());
     try t.expectEqualStrings("2023-06-01", cfg.anthropic_messages.api_version);
     try t.expectEqual(@as(u32, 64_000), cfg.anthropic_messages.max_tokens);
+    try t.expectEqual(Thinking.enabled, cfg.anthropic_messages.thinking);
+    try t.expectEqual(Effort.medium, cfg.anthropic_messages.effort);
+    try t.expectEqual(@as(?u32, 32_000), cfg.anthropic_messages.thinking_budget_tokens);
+    try t.expectEqual(false, cfg.anthropic_messages.thinking_interleaved);
+}
+
+test "ProviderConfig - anthropic_messages wireIdentity carries thinking fields" {
+    const cfg: ProviderConfig = .{ .anthropic_messages = .{
+        .api_key = "k",
+        .base_url = "https://api.anthropic.com",
+        .model = "claude-opus-4-8",
+        .thinking = .adaptive,
+        .effort = .high,
+        .thinking_budget_tokens = null,
+        .thinking_interleaved = false,
+    } };
+    const id = cfg.wireIdentity();
+    try t.expectEqual(APIStyle.anthropic_messages, id.api_style);
+    try t.expectEqual(Thinking.adaptive, id.thinking);
+    try t.expectEqual(Effort.high, id.effort);
+    try t.expectEqual(@as(?u32, null), id.thinking_budget_tokens);
+    try t.expectEqual(false, id.thinking_interleaved);
+    // reasoning field carries its zero default; not meaningful for Anthropic
+    try t.expectEqual(ReasoningEffort.default, id.reasoning);
 }
 
 test "ProviderConfig - openai_chat reasoning defaults to .default" {
diff --git a/libpanto/src/file_system_jsonl_store.zig b/libpanto/src/file_system_jsonl_store.zig
index bbf8ea5..c77d7d2 100644
--- a/libpanto/src/file_system_jsonl_store.zig
+++ b/libpanto/src/file_system_jsonl_store.zig
@@ -1351,6 +1351,10 @@ pub const FileSystemJSONLStore = struct {
                 .base_url = pm.identity.base_url,
                 .model = pm.identity.model,
                 .reasoning = pm.identity.reasoning,
+                .thinking = pm.identity.thinking,
+                .effort = pm.identity.effort,
+                .thinking_budget_tokens = pm.identity.thinking_budget_tokens,
+                .thinking_interleaved = pm.identity.thinking_interleaved,
             };
             built += 1;
         }
diff --git a/libpanto/src/provider_anthropic_messages.zig b/libpanto/src/provider_anthropic_messages.zig
index d76793b..8e41695 100644
--- a/libpanto/src/provider_anthropic_messages.zig
+++ b/libpanto/src/provider_anthropic_messages.zig
@@ -75,15 +75,31 @@ pub const AnthropicMessagesRequest = struct {
         const body = try json_mod.serializeRequest(self.allocator, self.config, conv, tools);
         defer self.allocator.free(body);
 
-        const extra_headers = [_]http.Header{
+        // Build headers. The four base headers are always present; the
+        // interleaved-thinking beta header is added only when the config
+        // explicitly requests manual extended thinking with interleaving.
+        // It is intentionally NOT sent for `.adaptive` (interleaving is
+        // automatic there and the header causes 400s on some backends) or
+        // `.disabled`.
+        var headers_buf: [5]http.Header = .{
             .{ .name = "content-type", .value = "application/json" },
             .{ .name = "accept", .value = "text/event-stream" },
             .{ .name = "x-api-key", .value = self.config.api_key },
             .{ .name = "anthropic-version", .value = self.config.api_version },
+            undefined, // slot reserved for the optional beta header
         };
+        const send_interleaved = self.config.thinking == .enabled and
+            self.config.thinking_interleaved;
+        if (send_interleaved) {
+            headers_buf[4] = .{
+                .name = "anthropic-beta",
+                .value = "interleaved-thinking-2025-05-14",
+            };
+        }
+        const extra_headers = headers_buf[0 .. if (send_interleaved) @as(usize, 5) else @as(usize, 4)];
 
         rr.req = try self.http_client.request(.POST, uri, .{
-            .extra_headers = &extra_headers,
+            .extra_headers = extra_headers,
             // Disable compression: gzip buffers small SSE frames, defeating
             // the streaming property we paid for `stream: true` to get.
             .headers = .{ .accept_encoding = .{ .override = "identity" } },
@@ -1095,3 +1111,56 @@ test "two streamed turns persist assistant replies in the conversation" {
         conv.messages.items[4].content.items[0].Text.items,
     );
 }
+
+/// Helper: build the header slice exactly as `open` does, given a config,
+/// and return whether the interleaved beta header is present.
+/// This lets us test the header-selection logic without a live HTTP connection.
+fn headerSliceIncludesInterleaved(cfg: *const config_mod.AnthropicMessagesConfig) bool {
+    const send_interleaved = cfg.thinking == .enabled and cfg.thinking_interleaved;
+    return send_interleaved;
+}
+
+test "interleaved beta header: enabled when thinking=.enabled and interleaved=true" {
+    const cfg: config_mod.AnthropicMessagesConfig = .{
+        .api_key = "k",
+        .base_url = "u",
+        .model = "m",
+        .thinking = .enabled,
+        .thinking_interleaved = true,
+    };
+    try testing.expect(headerSliceIncludesInterleaved(&cfg));
+}
+
+test "interleaved beta header: absent when thinking=.enabled and interleaved=false" {
+    const cfg: config_mod.AnthropicMessagesConfig = .{
+        .api_key = "k",
+        .base_url = "u",
+        .model = "m",
+        .thinking = .enabled,
+        .thinking_interleaved = false,
+    };
+    try testing.expect(!headerSliceIncludesInterleaved(&cfg));
+}
+
+test "interleaved beta header: absent when thinking=.adaptive even if interleaved=true" {
+    const cfg: config_mod.AnthropicMessagesConfig = .{
+        .api_key = "k",
+        .base_url = "u",
+        .model = "m",
+        .thinking = .adaptive,
+        .thinking_interleaved = true,
+    };
+    // .adaptive does not send the header; interleaving is automatic there.
+    try testing.expect(!headerSliceIncludesInterleaved(&cfg));
+}
+
+test "interleaved beta header: absent when thinking=.disabled" {
+    const cfg: config_mod.AnthropicMessagesConfig = .{
+        .api_key = "k",
+        .base_url = "u",
+        .model = "m",
+        .thinking = .disabled,
+        .thinking_interleaved = true,
+    };
+    try testing.expect(!headerSliceIncludesInterleaved(&cfg));
+}
diff --git a/libpanto/src/public.zig b/libpanto/src/public.zig
index 5488220..f9c2830 100644
--- a/libpanto/src/public.zig
+++ b/libpanto/src/public.zig
@@ -64,6 +64,8 @@ pub const OpenAIChatConfig = config_mod.OpenAIChatConfig;
 pub const AnthropicMessagesConfig = config_mod.AnthropicMessagesConfig;
 pub const APIStyle = config_mod.APIStyle;
 pub const ReasoningEffort = config_mod.ReasoningEffort;
+pub const Thinking = config_mod.Thinking;
+pub const Effort = config_mod.Effort;
 pub const CompactionConfig = config_mod.CompactionConfig;
 pub const RetryConfig = config_mod.RetryConfig;
 pub const WireIdentity = config_mod.WireIdentity;
diff --git a/libpanto/src/session.zig b/libpanto/src/session.zig
index ca7046c..492f94b 100644
--- a/libpanto/src/session.zig
+++ b/libpanto/src/session.zig
@@ -27,6 +27,8 @@ const config = @import("config.zig");
 
 pub const APIStyle = config.APIStyle;
 pub const ReasoningEffort = config.ReasoningEffort;
+pub const Thinking = config.Thinking;
+pub const Effort = config.Effort;
 
 /// Wire-format provider identity stamped on a message entry. This is the
 /// ground truth of which endpoint a turn was sent to — never a CLI config
@@ -36,7 +38,17 @@ pub const WireStamp = struct {
     api_style: APIStyle,
     base_url: []const u8, // owned
     model: []const u8, // owned
+    /// OpenAI only. Defaults to `.default` (field omitted on the wire).
     reasoning: ReasoningEffort = .default,
+    /// Anthropic only. Defaults to `.enabled`.
+    thinking: Thinking = .enabled,
+    /// Anthropic only; only meaningful when `thinking == .adaptive`.
+    effort: Effort = .medium,
+    /// Anthropic only; only meaningful when `thinking == .enabled`. `null`
+    /// means "use the config default" (falls back to `max_tokens - 1`).
+    thinking_budget_tokens: ?u32 = 32_000,
+    /// Anthropic only; only meaningful when `thinking == .enabled`.
+    thinking_interleaved: bool = false,
 
     pub fn deinit(self: WireStamp, alloc: Allocator) void {
         alloc.free(self.base_url);
@@ -52,6 +64,10 @@ pub const WireStamp = struct {
             .base_url = burl,
             .model = mdl,
             .reasoning = self.reasoning,
+            .thinking = self.thinking,
+            .effort = self.effort,
+            .thinking_budget_tokens = self.thinking_budget_tokens,
+            .thinking_interleaved = self.thinking_interleaved,
         };
     }
 };
@@ -338,8 +354,35 @@ fn writeWireStamp(s: *std.json.Stringify, st: WireStamp) !void {
     try s.write(st.base_url);
     try s.objectField("model");
     try s.write(st.model);
-    try s.objectField("reasoning");
-    try s.write(@tagName(st.reasoning));
+    // OpenAI: emit reasoning only when non-default (keeps logs compact).
+    if (st.reasoning != .default) {
+        try s.objectField("reasoning");
+        try s.write(@tagName(st.reasoning));
+    }
+    // Anthropic: emit thinking fields only when they differ from defaults.
+    if (st.thinking != .enabled) {
+        try s.objectField("thinking");
+        try s.write(@tagName(st.thinking));
+    }
+    if (st.effort != .medium) {
+        try s.objectField("effort");
+        try s.write(@tagName(st.effort));
+    }
+    if (st.thinking_budget_tokens) |b| {
+        if (b != 32_000) {
+            try s.objectField("thinkingBudgetTokens");
+            try s.write(b);
+        }
+    } else {
+        // null means "use max_tokens - 1"; record the absence explicitly
+        // so round-trips preserve the null intent.
+        try s.objectField("thinkingBudgetTokens");
+        try s.write(null);
+    }
+    if (st.thinking_interleaved) {
+        try s.objectField("thinkingInterleaved");
+        try s.write(true);
+    }
 }
 
 fn writeDiskMessage(s: *std.json.Stringify, msg: StoredMessage) !void {
@@ -572,12 +615,45 @@ fn parseWireStamp(allocator: Allocator, obj: std.json.ObjectMap) ParseError!?Wir
     errdefer allocator.free(base_url);
     const model = try dupeStringField(allocator, obj, "model");
     errdefer allocator.free(model);
+    // OpenAI: absent reasoning defaults to .default.
     const reasoning: ReasoningEffort = blk: {
         const rv = obj.get("reasoning") orelse break :blk .default;
         if (rv != .string) break :blk .default;
         break :blk std.meta.stringToEnum(ReasoningEffort, rv.string) orelse .default;
     };
-    return .{ .api_style = api_style, .base_url = base_url, .model = model, .reasoning = reasoning };
+    // Anthropic: absent fields default to the same values as the config defaults.
+    const thinking: Thinking = blk: {
+        const tv = obj.get("thinking") orelse break :blk .enabled;
+        if (tv != .string) break :blk .enabled;
+        break :blk std.meta.stringToEnum(Thinking, tv.string) orelse .enabled;
+    };
+    const effort: Effort = blk: {
+        const ev = obj.get("effort") orelse break :blk .medium;
+        if (ev != .string) break :blk .medium;
+        break :blk std.meta.stringToEnum(Effort, ev.string) orelse .medium;
+    };
+    const thinking_budget_tokens: ?u32 = blk: {
+        const bv = obj.get("thinkingBudgetTokens") orelse break :blk 32_000;
+        if (bv == .null) break :blk null;
+        if (bv != .integer) break :blk 32_000;
+        if (bv.integer < 0) break :blk 32_000;
+        break :blk @intCast(bv.integer);
+    };
+    const thinking_interleaved: bool = blk: {
+        const iv = obj.get("thinkingInterleaved") orelse break :blk false;
+        if (iv != .bool) break :blk false;
+        break :blk iv.bool;
+    };
+    return .{
+        .api_style = api_style,
+        .base_url = base_url,
+        .model = model,
+        .reasoning = reasoning,
+        .thinking = thinking,
+        .effort = effort,
+        .thinking_budget_tokens = thinking_budget_tokens,
+        .thinking_interleaved = thinking_interleaved,
+    };
 }
 
 fn parseDiskMessage(allocator: Allocator, obj: std.json.ObjectMap) ParseError!StoredMessage {
@@ -1366,3 +1442,148 @@ test "compactionSummary bridges in-memory <-> disk both directions" {
     defer inmem.deinit(a);
     try testing.expectEqualStrings("S1", inmem.CompactionSummary.text.items);
 }
+
+test "WireStamp: Anthropic non-default thinking fields round-trip" {
+    const a = testing.allocator;
+
+    var content = try a.alloc(StoredContentBlock, 1);
+    content[0] = .{ .text = .{ .text = try dupe(a, "hi") } };
+
+    const entry: SessionEntry = .{ .message = .{
+        .base = .{
+            .id = try dupe(a, "aa000001"),
+            .parent_id = null,
+            .timestamp = try dupe(a, "2026-06-01T00:00:00Z"),
+        },
+        .stamp = .{
+            .api_style = .anthropic_messages,
+            .base_url = try dupe(a, "https://api.anthropic.com"),
+            .model = try dupe(a, "claude-opus-4-8"),
+            .thinking = .adaptive,
+            .effort = .high,
+            .thinking_budget_tokens = null,
+            .thinking_interleaved = true,
+        },
+        .message = .{ .role = .user, .content = content },
+    } };
+    defer entry.deinit(a);
+
+    const line = try serializeEntry(a, entry);
+    defer a.free(line);
+
+    // Non-default fields must appear in the serialized line.
+    try testing.expect(std.mem.indexOf(u8, line, "\"thinking\":\"adaptive\"") != null);
+    try testing.expect(std.mem.indexOf(u8, line, "\"effort\":\"high\"") != null);
+    try testing.expect(std.mem.indexOf(u8, line, "\"thinkingBudgetTokens\":null") != null);
+    try testing.expect(std.mem.indexOf(u8, line, "\"thinkingInterleaved\":true") != null);
+
+    var fe = try parseLine(a, line);
+    defer fe.deinit(a);
+    const got = fe.entry.message.stamp.?;
+    try testing.expectEqual(APIStyle.anthropic_messages, got.api_style);
+    try testing.expectEqual(Thinking.adaptive, got.thinking);
+    try testing.expectEqual(Effort.high, got.effort);
+    try testing.expectEqual(@as(?u32, null), got.thinking_budget_tokens);
+    try testing.expectEqual(true, got.thinking_interleaved);
+    // reasoning carries its default (unused for Anthropic)
+    try testing.expectEqual(ReasoningEffort.default, got.reasoning);
+}
+
+test "WireStamp: Anthropic stamp with all-default thinking fields omits non-essential keys" {
+    const a = testing.allocator;
+
+    var content = try a.alloc(StoredContentBlock, 1);
+    content[0] = .{ .text = .{ .text = try dupe(a, "hi") } };
+
+    const entry: SessionEntry = .{ .message = .{
+        .base = .{
+            .id = try dupe(a, "bb000002"),
+            .parent_id = null,
+            .timestamp = try dupe(a, "2026-06-01T00:00:00Z"),
+        },
+        .stamp = .{
+            .api_style = .anthropic_messages,
+            .base_url = try dupe(a, "https://api.anthropic.com"),
+            .model = try dupe(a, "claude-haiku-4-5"),
+            // All defaults: thinking=.enabled, effort=.medium,
+            // thinking_budget_tokens=32_000, thinking_interleaved=false
+        },
+        .message = .{ .role = .user, .content = content },
+    } };
+    defer entry.deinit(a);
+
+    const line = try serializeEntry(a, entry);
+    defer a.free(line);
+
+    // Default-valued fields should be omitted (keeps logs compact).
+    try testing.expect(std.mem.indexOf(u8, line, "thinking") == null);
+    try testing.expect(std.mem.indexOf(u8, line, "effort") == null);
+    try testing.expect(std.mem.indexOf(u8, line, "thinkingInterleaved") == null);
+    // thinkingBudgetTokens=32_000 is the default, should be omitted too.
+    try testing.expect(std.mem.indexOf(u8, line, "thinkingBudgetTokens") == null);
+
+    // Round-trip: all defaults parse back correctly.
+    var fe = try parseLine(a, line);
+    defer fe.deinit(a);
+    const got = fe.entry.message.stamp.?;
+    try testing.expectEqual(Thinking.enabled, got.thinking);
+    try testing.expectEqual(Effort.medium, got.effort);
+    try testing.expectEqual(@as(?u32, 32_000), got.thinking_budget_tokens);
+    try testing.expectEqual(false, got.thinking_interleaved);
+}
+
+test "WireStamp: legacy Anthropic stamp (no thinking fields) parses with defaults" {
+    // Simulate a session log written before thinking fields were added.
+    const a = testing.allocator;
+    const line =
+        \\{"type":"message","id":"cc000003","parentId":null,"timestamp":"2026-06-01T00:00:00Z","apiStyle":"anthropic_messages","baseUrl":"https://api.anthropic.com","model":"claude-3-7-sonnet","message":{"role":"user","content":[{"type":"text","text":"hi"}]}}
+    ;
+    var fe = try parseLine(a, line);
+    defer fe.deinit(a);
+    const got = fe.entry.message.stamp.?;
+    try testing.expectEqual(APIStyle.anthropic_messages, got.api_style);
+    try testing.expectEqual(Thinking.enabled, got.thinking);
+    try testing.expectEqual(Effort.medium, got.effort);
+    try testing.expectEqual(@as(?u32, 32_000), got.thinking_budget_tokens);
+    try testing.expectEqual(false, got.thinking_interleaved);
+}
+
+test "WireStamp: OpenAI stamp is unchanged by Anthropic fields" {
+    const a = testing.allocator;
+
+    var content = try a.alloc(StoredContentBlock, 1);
+    content[0] = .{ .text = .{ .text = try dupe(a, "hi") } };
+
+    const entry: SessionEntry = .{ .message = .{
+        .base = .{
+            .id = try dupe(a, "dd000004"),
+            .parent_id = null,
+            .timestamp = try dupe(a, "2026-06-01T00:00:00Z"),
+        },
+        .stamp = .{
+            .api_style = .openai_chat,
+            .base_url = try dupe(a, "https://api.openai.com/v1"),
+            .model = try dupe(a, "gpt-4o"),
+            .reasoning = .high,
+        },
+        .message = .{ .role = .user, .content = content },
+    } };
+    defer entry.deinit(a);
+
+    const line = try serializeEntry(a, entry);
+    defer a.free(line);
+
+    // Anthropic fields should not appear for an OpenAI stamp.
+    try testing.expect(std.mem.indexOf(u8, line, "thinking") == null);
+    try testing.expect(std.mem.indexOf(u8, line, "effort") == null);
+    try testing.expect(std.mem.indexOf(u8, line, "thinkingBudget") == null);
+    try testing.expect(std.mem.indexOf(u8, line, "thinkingInterleaved") == null);
+    // reasoning=high should be present
+    try testing.expect(std.mem.indexOf(u8, line, "\"reasoning\":\"high\"") != null);
+
+    var fe = try parseLine(a, line);
+    defer fe.deinit(a);
+    const got = fe.entry.message.stamp.?;
+    try testing.expectEqual(APIStyle.openai_chat, got.api_style);
+    try testing.expectEqual(ReasoningEffort.high, got.reasoning);
+}