feat(provider): Anthropic Messages provider

src/acp/server.zig

@@ -353,6 +353,7 @@ pub fn selectCredentialForProvider(
             .refresh_if_needed,
             provider,
             state.credential_source,
+            null,
         );
         break :blk resolution.credential orelse return false;
     };
@@ -1370,6 +1371,7 @@ fn handleInitialize(state: *ServerState, alloc: Allocator, msg: *jsonrpc.Message
             .refresh_if_needed,
             state.provider,
             preferred,
+            null,
         );
         routed_credential = resolution.credential;
         if (routed_credential == null) {
@@ -1651,6 +1653,7 @@ fn handleSetConfigOption(state: *ServerState, alloc: Allocator, msg: *jsonrpc.Me
                     .refresh_if_needed,
                     target,
                     null,
+                    null,
                 );
                 break :credential resolution.credential orelse
                     return state.writer.writeError(alloc, msg.id, .{

src/builtins/providers.zig

@@ -6,6 +6,8 @@ const openai_codex_permission_reviewer = @import("../gateway/openai_codex_permis
 const xai_grok = @import("../gateway/xai_grok.zig");
 const xai_grok_models = @import("../gateway/xai_grok_models.zig");
 const xai_grok_permission_reviewer = @import("../gateway/xai_grok_permission_reviewer.zig");
+const anthropic = @import("../gateway/anthropic.zig");
+const anthropic_models = @import("../gateway/anthropic_models.zig");
 const provider_catalog = @import("../core/auth/provider_catalog.zig");
 
 pub const native = provider_set.Set{
@@ -26,4 +28,10 @@ pub const native = provider_set.Set{
         .model_catalog = xai_grok_models.model_catalog_provider,
         .permission_reviewer = xai_grok_permission_reviewer.provider,
     },
+    .anthropic = .{
+        .presentation = provider_catalog.find(.anthropic),
+        .agent_stream = anthropic.agent_stream_provider,
+        .cli_model_catalog = anthropic_models.cli_model_catalog_provider,
+        .model_catalog = anthropic_models.model_catalog_provider,
+    },
 };

src/core/app/app_agent_runtime.zig

@@ -1064,6 +1064,10 @@ pub fn Runtime(comptime App: type) type {
                         .agent_stream = tool_context.agent_stream_provider,
                         .permission_reviewer = tool_context.permission_reviewer_provider,
                     },
+                    .anthropic = .{
+                        .agent_stream = tool_context.agent_stream_provider,
+                        .permission_reviewer = null,
+                    },
                 };
             return subagent_agent_adapter.run(.{
                 .host = app_session_runtime.Runtime(App).subagentHost(app) orelse

src/core/app/app_auth_runtime.zig

@@ -59,6 +59,7 @@ pub fn Runtime(comptime App: type) type {
                 const required_source: credentials.Source = switch (provider) {
                     .codex => .chatgpt_subscription,
                     .grok => .grok_subscription,
+                    .anthropic => .anthropic_api_key,
                     .gateway => app.auth.credentialSource() orelse .fx_login,
                 };
                 const route_change = app.auth.selectForProvider(app.alloc, provider) catch |err| switch (err) {
@@ -778,6 +779,7 @@ pub fn Runtime(comptime App: type) type {
                 .refresh_if_needed,
                 target,
                 null,
+                null,
             ) catch |err| {
                 debug_trace.logf("provider", "credential preparation failed provider={t} err={s}", .{ target, @errorName(err) });
                 try app.writeDomainNotice(.{
@@ -1397,7 +1399,7 @@ test "interactive subscription sign-in rejects active and queued work before OAu
             switch (provider) {
                 .codex => try Runtime(BusySignInApp).beginChatGptSignIn(&app),
                 .grok => try Runtime(BusySignInApp).beginGrokSignIn(&app),
-                .gateway => unreachable,
+                .gateway, .anthropic => unreachable,
             }
 
             try std.testing.expectEqual(@as(usize, 0), app.auth.start_count);

src/core/app/app_lifecycle.zig

@@ -149,6 +149,13 @@ pub const StartupState = struct {
     notification_attention_required: bool = false,
     notification_max: bool = false,
     theme_monitor_enabled: bool = false,
+    profile_anthropic_api_key: []u8 = &.{},
+
+    pub fn takeProfileAnthropicApiKey(self: *StartupState) []u8 {
+        const value = self.profile_anthropic_api_key;
+        self.profile_anthropic_api_key = &.{};
+        return value;
+    }
 
     pub fn deinit(self: *StartupState, alloc: Allocator) void {
         self.workspace_access.deinit(alloc);
@@ -400,6 +407,9 @@ fn loadStartupStateFromOwnedWorkspace(
     state.model_source = detailed.model_source orelse .compiled_default;
     state.selected_model = try loadInitialModel(alloc, configured_selection.model, null);
     if (hasProcessModelOverride()) state.model_source = .process_override;
+    if (settings.anthropic_api_key) |profile_key| {
+        state.profile_anthropic_api_key = try alloc.dupe(u8, profile_key);
+    }
     state.config_diagnostics = detailed.diagnostics;
     detailed.diagnostics = &.{};
     state.prompt_history_enabled = settings.prompt_history_enabled orelse true;
@@ -412,6 +422,7 @@ fn loadStartupStateFromOwnedWorkspace(
             mode,
             state.provider,
             settings.credential_source,
+            settings.anthropic_api_key,
         );
         state.credential = resolution.credential;
         state.stored_key_status = resolution.stored_key_status;
@@ -1113,12 +1124,22 @@ fn configuredProviderSelection(
     const provider = settings.provider orelse .gateway;
     const model = settings.models.get(provider) orelse switch (provider) {
         .gateway => default_model,
-        .codex => return error.CodexModelNotSelected,
-        .grok => return error.GrokModelNotSelected,
+        .codex, .grok, .anthropic => switchModelFromEnv() orelse switch (provider) {
+            .codex => return error.CodexModelNotSelected,
+            .grok => return error.GrokModelNotSelected,
+            .anthropic => return error.AnthropicModelNotSelected,
+            .gateway => unreachable,
+        },
     };
     return .{ .provider = provider, .model = model };
 }
 
+fn switchModelFromEnv() ?[]const u8 {
+    const raw = io_mod.getenv("FX_MODEL") orelse return null;
+    const trimmed = std.mem.trim(u8, raw, " \t\r\n");
+    return if (trimmed.len > 0) trimmed else null;
+}
+
 fn initialModelId(default_model: []const u8, configured: ?[]const u8) []const u8 {
     const model = io_mod.getenv("FX_MODEL") orelse return configured orelse default_model;
     const trimmed = std.mem.trim(u8, model, " \t\r\n");
@@ -1151,6 +1172,17 @@ test "startup provider chooses only its provider-scoped model" {
     const grok = try configuredProviderSelection("default/model", &grok_settings);
     try std.testing.expectEqual(model_provider.ProviderId.grok, grok.provider);
     try std.testing.expectEqualStrings("grok-model", grok.model);
+    var anthropic_settings = config_runtime.Settings{ .provider = .anthropic };
+    anthropic_settings.models.values[@intFromEnum(model_provider.ProviderId.anthropic)] = @constCast("claude-opus-5");
+    const anthropic = try configuredProviderSelection("default/model", &anthropic_settings);
+    try std.testing.expectEqual(model_provider.ProviderId.anthropic, anthropic.provider);
+    try std.testing.expectEqualStrings("claude-opus-5", anthropic.model);
+
+    const missing_anthropic = config_runtime.Settings{ .provider = .anthropic };
+    try std.testing.expectError(
+        error.AnthropicModelNotSelected,
+        configuredProviderSelection("default/model", &missing_anthropic),
+    );
 }
 
 fn loadInitialModel(alloc: Allocator, default_model: []const u8, configured: ?[]const u8) ![]u8 {

src/core/auth/auth_runtime.zig

@@ -678,6 +678,7 @@ pub fn loadStatusSnapshotForProvider(
             .stored,
             selected_provider,
             preferred,
+            null,
         )
     else
         credentials.resolvePreferring(
@@ -1636,6 +1637,15 @@ pub const Runtime = struct {
                     self,
                     loadRuntimeCredentialSource,
                 ),
+            .anthropic => if (self.credentialSource() == .anthropic_api_key)
+                false
+            else
+                self.selectSourceWithLoader(
+                    alloc,
+                    .anthropic_api_key,
+                    self,
+                    loadRuntimeCredentialSource,
+                ),
             .gateway => if (self.credentialSource() != .chatgpt_subscription and self.credentialSource() != .grok_subscription)
                 false
             else

src/core/auth/auth_transition.zig

@@ -73,6 +73,7 @@ pub fn signInCompletion(
             .{ .switch_provider = .grok }
         else
             .{ .activate_source = .grok_subscription },
+        .anthropic => .{ .activate_source = .anthropic_api_key },
     };
 }
 

src/core/auth/credential_authority.zig

@@ -25,6 +25,7 @@ pub fn derive(
         .ai_gateway_api_key,
         .fx_login,
         .stored_key,
+        .anthropic_api_key,
         => hash.update("\x00slot\x00"),
         .chatgpt_subscription,
         .grok_subscription,