initial commit, moved libpanto over from the pantograph repo

1 files changed, 954 insertions, 0 deletions

diff --git a/src/auth.zig b/src/auth.zig
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--- /dev/null
+++ b/src/auth.zig
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+//! Provider authentication: named auth sessions resolved into request-ready
+//! credentials before a provider stream opens.
+//!
+//! A provider names the auth session it uses (`auth = "<name>"`); core
+//! resolves that session into a `ResolvedCredential` (a bearer/api-key value,
+//! an optional dynamic `base_url`, and optional auth-derived request headers).
+//! This covers three families with one configuration shape:
+//!
+//!   - `api_key`     — static key from a literal or an environment variable.
+//!   - `oauth_device` — OAuth 2.0 device-authorization flow. Two dialects:
+//!       * `token` — standard device flow (GitHub Copilot). The poll endpoint
+//!         returns the OAuth token response directly.
+//!       * `codex` — OpenAI Codex device flow. The poll endpoint returns an
+//!         authorization code plus a server-generated PKCE verifier; core
+//!         exchanges those at `token_url` for `{access,refresh,id}_token`.
+//!
+//! This module owns the *mechanism* (config types, the persisted token shape,
+//! and — added incrementally — the HTTP flows and refresh lifecycle). It is
+//! UI- and filesystem-policy-agnostic: token storage takes a directory path,
+//! and interactive device-code prompts are delivered through a caller-supplied
+//! `Presenter`. That keeps libpanto reusable while the embedder owns token
+//! storage policy and how a device code is shown to the user.
+
+const std = @import("std");
+const builtin = @import("builtin");
+const Io = std.Io;
+const config_mod = @import("config.zig");
+const http = @import("http_helper.zig");
+
+/// A single request header. Re-exported from `config.zig` so callers can
+/// build auth-derived headers and provider `extra_headers` from one type.
+pub const Header = config_mod.Header;
+
+// ===========================================================================
+// Auth configuration (parsed from `[auth.<name>]`)
+// ===========================================================================
+
+pub const AuthType = enum { api_key, oauth_device };
+
+/// Device-flow completion shape. See the module doc.
+pub const DeviceDialect = enum { token, codex };
+
+/// Wire encoding for the `token` dialect's device-code / poll request bodies.
+pub const TokenRequestFormat = enum { form, json };
+
+/// Static API-key auth. `key` is the resolved credential — a literal, or the
+/// result of `${env:VAR}`-style substitution performed by the embedder. An
+/// absent/empty key means unresolved; providers using such a session are
+/// omitted from the active config (export the key and they reappear).
+pub const ApiKeyAuth = struct {
+    key: ?[]const u8 = null,
+};
+
+/// A secondary token exchange run after the durable OAuth token is obtained
+/// (GitHub Copilot's `/copilot_internal/v2/token`). The exchange result —
+/// not the OAuth token — becomes the request credential, and may also
+/// override the provider `base_url`.
+pub const ExchangeConfig = struct {
+    method: []const u8 = "GET",
+    url: []const u8,
+    /// Which stored token to send as the exchange request's bearer.
+    bearer: BearerSource = .oauth_access_token,
+    /// Dotted JSON path to the credential in the exchange response.
+    token_json_path: []const u8 = "token",
+    /// Dotted JSON path to the credential's unix-seconds expiry, if any.
+    expires_at_json_path: ?[]const u8 = null,
+    /// Dotted JSON path to a dynamic `base_url` in the exchange response.
+    base_url_json_path: ?[]const u8 = null,
+
+    pub const BearerSource = enum { oauth_access_token };
+};
+
+/// OAuth 2.0 device-authorization auth.
+pub const OAuthDeviceAuth = struct {
+    dialect: DeviceDialect = .token,
+    client_id: []const u8,
+    /// Endpoint that issues the device/user code.
+    device_code_url: []const u8,
+    /// `token` dialect: poll-for-token and refresh endpoint.
+    /// `codex` dialect: authorization-code exchange and refresh endpoint.
+    token_url: []const u8,
+    /// `codex` dialect: endpoint polled for the authorization code.
+    device_poll_url: ?[]const u8 = null,
+    /// Browser URL shown to the user (codex). For the `token` dialect the
+    /// verification URI comes from the device-code response.
+    verification_url: ?[]const u8 = null,
+    scope: ?[]const u8 = null,
+    /// Request encoding for the `token` dialect device-code / poll bodies.
+    token_request_format: TokenRequestFormat = .form,
+    /// `codex` dialect: redirect URI sent in the code exchange.
+    redirect_uri: ?[]const u8 = null,
+    /// `codex` dialect: JWT claim (on the id_token) holding the account id,
+    /// e.g. `https://api.openai.com/auth`. When set, the resolver extracts an
+    /// account id and emits it as a header (see provider wiring).
+    account_id_jwt_claim: ?[]const u8 = null,
+    /// Optional post-login token exchange (Copilot).
+    exchange: ?ExchangeConfig = null,
+};
+
+/// One named auth session's configuration. The union tag mirrors the TOML
+/// `type` field.
+pub const AuthConfig = union(AuthType) {
+    api_key: ApiKeyAuth,
+    oauth_device: OAuthDeviceAuth,
+};
+
+// ===========================================================================
+// Persisted token state (`<auth_dir>/<name>.json`)
+// ===========================================================================
+
+/// Durable auth state for one session. Only the fields relevant to the auth
+/// type are populated. Treat the on-disk file like a password.
+pub const TokenSet = struct {
+    /// `"api_key"` | `"oauth_device"` — records which family wrote the file.
+    type: []const u8 = "oauth_device",
+    /// Durable OAuth access token (the `ghu_...` for Copilot; the JWT access
+    /// token for Codex).
+    access_token: ?[]const u8 = null,
+    refresh_token: ?[]const u8 = null,
+    id_token: ?[]const u8 = null,
+    /// Unix-seconds expiry of `access_token` (when known; OAuth `expires_in`
+    /// or a JWT `exp`). Null for tokens with no intrinsic expiry (Copilot's
+    /// durable `ghu_` token).
+    expires_at: ?i64 = null,
+    /// Account id derived from the id_token (Codex `chatgpt-account-id`).
+    account_id: ?[]const u8 = null,
+    /// Result of the secondary exchange (Copilot short-lived API token).
+    exchange: ?ExchangeToken = null,
+
+    pub const ExchangeToken = struct {
+        token: []const u8,
+        /// Unix-seconds expiry of the exchanged token.
+        expires_at: ?i64 = null,
+        /// Dynamic base_url returned by the exchange, if any.
+        base_url: ?[]const u8 = null,
+    };
+};
+
+// ===========================================================================
+// Resolved credential (handed to the provider for one turn)
+// ===========================================================================
+
+/// The request-ready output of resolving an auth session: the secret to place
+/// in the provider's auth header, plus any dynamic base_url and auth-derived
+/// headers. The embedder injects these into the active `ProviderConfig`.
+pub const ResolvedCredential = struct {
+    /// Value for the provider auth header (the bearer / x-api-key value).
+    api_key: []const u8,
+    /// Overrides the provider's configured `base_url` when present (e.g.
+    /// Copilot's `endpoints.api`).
+    base_url_override: ?[]const u8 = null,
+    /// Auth-derived headers (e.g. `chatgpt-account-id`) to merge onto the
+    /// provider's request.
+    extra_headers: []const Header = &.{},
+};
+
+// ===========================================================================
+// Token storage (`<auth_dir>/<name>.json`)
+// ===========================================================================
+
+/// A parsed `TokenSet` plus the arena owning its strings. Call `deinit`.
+pub const ParsedTokenSet = std.json.Parsed(TokenSet);
+
+/// Owner-only file permissions for token files (POSIX 0o600). On Windows the
+/// platform `Permissions` enum has no mode, so fall back to its default.
+fn tokenFilePermissions() Io.File.Permissions {
+    if (builtin.os.tag == .windows) return .default_file;
+    return Io.File.Permissions.fromMode(0o600);
+}
+
+/// Load and parse `<auth_dir>/<name>.json`. Returns null if the file does not
+/// exist. The caller owns the result and must `deinit` it.
+pub fn loadTokenSet(
+    allocator: std.mem.Allocator,
+    io: Io,
+    auth_dir: []const u8,
+    name: []const u8,
+) !?ParsedTokenSet {
+    const fname = try std.fmt.allocPrint(allocator, "{s}.json", .{name});
+    defer allocator.free(fname);
+
+    var dir = Io.Dir.cwd().openDir(io, auth_dir, .{}) catch |err| switch (err) {
+        error.FileNotFound => return null,
+        else => return err,
+    };
+    defer dir.close(io);
+
+    const bytes = dir.readFileAlloc(io, fname, allocator, .limited(1 << 20)) catch |err| switch (err) {
+        error.FileNotFound => return null,
+        else => return err,
+    };
+    defer allocator.free(bytes);
+
+    return try std.json.parseFromSlice(TokenSet, allocator, bytes, .{
+        .ignore_unknown_fields = true,
+        .allocate = .alloc_always,
+    });
+}
+
+/// Serialize and write `ts` to `<auth_dir>/<name>.json` with owner-only
+/// permissions, creating `auth_dir` if needed. Null optional fields are
+/// omitted to keep the file tidy.
+pub fn saveTokenSet(
+    allocator: std.mem.Allocator,
+    io: Io,
+    auth_dir: []const u8,
+    name: []const u8,
+    ts: TokenSet,
+) !void {
+    Io.Dir.cwd().createDirPath(io, auth_dir) catch |err| switch (err) {
+        error.PathAlreadyExists => {},
+        else => return err,
+    };
+
+    const json = try std.json.Stringify.valueAlloc(allocator, ts, .{
+        .emit_null_optional_fields = false,
+        .whitespace = .indent_2,
+    });
+    defer allocator.free(json);
+
+    const fname = try std.fmt.allocPrint(allocator, "{s}.json", .{name});
+    defer allocator.free(fname);
+
+    var dir = try Io.Dir.cwd().openDir(io, auth_dir, .{});
+    defer dir.close(io);
+    try dir.writeFile(io, .{
+        .sub_path = fname,
+        .data = json,
+        .flags = .{ .permissions = tokenFilePermissions() },
+    });
+}
+
+/// Delete `<auth_dir>/<name>.json`. Returns true if a file was removed, false
+/// if it did not exist (idempotent logout).
+pub fn deleteTokenSet(
+    allocator: std.mem.Allocator,
+    io: Io,
+    auth_dir: []const u8,
+    name: []const u8,
+) !bool {
+    const fname = try std.fmt.allocPrint(allocator, "{s}.json", .{name});
+    defer allocator.free(fname);
+    var dir = Io.Dir.cwd().openDir(io, auth_dir, .{}) catch |err| switch (err) {
+        error.FileNotFound => return false,
+        else => return err,
+    };
+    defer dir.close(io);
+    dir.deleteFile(io, fname) catch |err| switch (err) {
+        error.FileNotFound => return false,
+        else => return err,
+    };
+    return true;
+}
+
+// ===========================================================================
+// OAuth device flow
+// ===========================================================================
+
+pub const AuthError = error{
+    /// No persisted token and no interactive presenter to run a login.
+    AuthLoginRequired,
+    /// The OAuth endpoint returned an error (denied, expired, malformed).
+    AuthFlowFailed,
+    /// The device-flow poll exceeded its deadline.
+    AuthLoginTimeout,
+    /// The configured auth could not be resolved into a credential.
+    AuthUnresolved,
+};
+
+/// The device-code prompt shown to the user. The embedder renders this
+/// (a verification URL + a short user code) however suits its UI.
+pub const DeviceCodePrompt = struct {
+    verification_uri: []const u8,
+    user_code: []const u8,
+    /// Seconds until the code expires (0 if the endpoint didn't say).
+    expires_in: i64 = 0,
+};
+
+/// Caller-supplied UI hook for interactive device login. `onDeviceCode` is
+/// called once with the prompt; `onStatus` (optional) receives progress text.
+pub const Presenter = struct {
+    ptr: *anyopaque,
+    vtable: *const VTable,
+
+    pub const VTable = struct {
+        on_device_code: *const fn (ptr: *anyopaque, prompt: DeviceCodePrompt) void,
+        on_status: ?*const fn (ptr: *anyopaque, msg: []const u8) void = null,
+    };
+
+    fn deviceCode(self: Presenter, prompt: DeviceCodePrompt) void {
+        self.vtable.on_device_code(self.ptr, prompt);
+    }
+    fn status(self: Presenter, msg: []const u8) void {
+        if (self.vtable.on_status) |f| f(self.ptr, msg);
+    }
+};
+
+/// The OAuth token-endpoint response, fields borrowed from an arena.
+pub const OAuthTokens = struct {
+    access_token: []const u8,
+    refresh_token: ?[]const u8 = null,
+    id_token: ?[]const u8 = null,
+    /// Seconds-to-live from `expires_in`, if present.
+    expires_in: ?i64 = null,
+};
+
+/// Identifiers needed to poll a started device authorization.
+const DeviceAuth = struct {
+    /// `token` dialect poll parameter.
+    device_code: ?[]const u8 = null,
+    /// `codex` dialect poll parameter.
+    device_auth_id: ?[]const u8 = null,
+    user_code: []const u8,
+    verification_uri: []const u8,
+    interval_secs: u32 = 5,
+    expires_in: i64 = 0,
+};
+
+/// `application/x-www-form-urlencoded` body from name/value pairs. Values are
+/// percent-encoded (unreserved chars pass through). Allocated in `arena`.
+fn formEncode(arena: std.mem.Allocator, pairs: []const [2][]const u8) ![]u8 {
+    var out: std.ArrayList(u8) = .empty;
+    for (pairs, 0..) |kv, i| {
+        if (i != 0) try out.append(arena, '&');
+        try percentEncode(arena, &out, kv[0]);
+        try out.append(arena, '=');
+        try percentEncode(arena, &out, kv[1]);
+    }
+    return out.toOwnedSlice(arena);
+}
+
+fn percentEncode(arena: std.mem.Allocator, out: *std.ArrayList(u8), s: []const u8) !void {
+    for (s) |c| {
+        const unreserved = (c >= 'A' and c <= 'Z') or (c >= 'a' and c <= 'z') or
+            (c >= '0' and c <= '9') or c == '-' or c == '.' or c == '_' or c == '~';
+        if (unreserved) {
+            try out.append(arena, c);
+        } else {
+            try out.print(arena, "%{X:0>2}", .{c});
+        }
+    }
+}
+
+/// Build the device-code / token request body in the dialect's encoding.
+fn encodeBody(
+    arena: std.mem.Allocator,
+    format: TokenRequestFormat,
+    pairs: []const [2][]const u8,
+) !struct { body: []u8, content_type: []const u8 } {
+    switch (format) {
+        .form => return .{ .body = try formEncode(arena, pairs), .content_type = "application/x-www-form-urlencoded" },
+        .json => {
+            var aw: std.Io.Writer.Allocating = .init(arena);
+            var s: std.json.Stringify = .{ .writer = &aw.writer };
+            try s.beginObject();
+            for (pairs) |kv| {
+                try s.objectField(kv[0]);
+                try s.write(kv[1]);
+            }
+            try s.endObject();
+            return .{ .body = aw.written(), .content_type = "application/json" };
+        },
+    }
+}
+
+/// Request a device/user code. `headers` are the client-identity headers
+/// (the provider's `extra_headers`) sent on every auth HTTP call. Allocates
+/// the result into `arena`.
+fn requestDeviceAuth(
+    arena: std.mem.Allocator,
+    client: *std.http.Client,
+    oauth: OAuthDeviceAuth,
+    headers: []const Header,
+) !DeviceAuth {
+    const enc = switch (oauth.dialect) {
+        // GitHub: `client_id` (+ optional scope), in the configured encoding.
+        .token => try encodeBody(arena, oauth.token_request_format, blk: {
+            if (oauth.scope) |sc| {
+                break :blk &[_][2][]const u8{ .{ "client_id", oauth.client_id }, .{ "scope", sc } };
+            } else break :blk &[_][2][]const u8{.{ "client_id", oauth.client_id }};
+        }),
+        // Codex: JSON `{client_id}`.
+        .codex => try encodeBody(arena, .json, &.{.{ "client_id", oauth.client_id }}),
+    };
+
+    const resp = try http.request(arena, client, .POST, oauth.device_code_url, .{
+        .headers = headers,
+        .body = enc.body,
+        .content_type = enc.content_type,
+    });
+    if (!resp.ok()) return AuthError.AuthFlowFailed;
+
+    const parsed = std.json.parseFromSlice(std.json.Value, arena, resp.body, .{}) catch
+        return AuthError.AuthFlowFailed;
+    const v = parsed.value;
+
+    const user_code = http.jsonStringAtPath(v, "user_code") orelse
+        http.jsonStringAtPath(v, "usercode") orelse return AuthError.AuthFlowFailed;
+    const interval: u32 = blk: {
+        if (http.jsonIntAtPath(v, "interval")) |iv| break :blk @intCast(@max(iv, 1));
+        // Codex returns interval as a string.
+        if (http.jsonStringAtPath(v, "interval")) |is| {
+            if (std.fmt.parseInt(i64, is, 10) catch null) |iv| break :blk @intCast(@max(iv, 1));
+        }
+        break :blk 5;
+    };
+    const verification_uri = switch (oauth.dialect) {
+        .token => http.jsonStringAtPath(v, "verification_uri") orelse
+            http.jsonStringAtPath(v, "verification_uri_complete") orelse return AuthError.AuthFlowFailed,
+        .codex => oauth.verification_url orelse return AuthError.AuthFlowFailed,
+    };
+    return .{
+        .device_code = http.jsonStringAtPath(v, "device_code"),
+        .device_auth_id = http.jsonStringAtPath(v, "device_auth_id"),
+        .user_code = user_code,
+        .verification_uri = verification_uri,
+        .interval_secs = interval,
+        .expires_in = http.jsonIntAtPath(v, "expires_in") orelse 0,
+    };
+}
+
+const PollResult = union(enum) {
+    pending,
+    done: OAuthTokens,
+};
+
+/// Poll once for completion. Allocates any returned tokens into `arena`.
+fn pollOnce(
+    arena: std.mem.Allocator,
+    client: *std.http.Client,
+    oauth: OAuthDeviceAuth,
+    da: DeviceAuth,
+    headers: []const Header,
+) !PollResult {
+    switch (oauth.dialect) {
+        .token => {
+            const enc = try encodeBody(arena, oauth.token_request_format, &.{
+                .{ "client_id", oauth.client_id },
+                .{ "device_code", da.device_code orelse return AuthError.AuthFlowFailed },
+                .{ "grant_type", "urn:ietf:params:oauth:grant-type:device_code" },
+            });
+            const resp = try http.request(arena, client, .POST, oauth.token_url, .{
+                .headers = headers,
+                .body = enc.body,
+                .content_type = enc.content_type,
+            });
+            const parsed = std.json.parseFromSlice(std.json.Value, arena, resp.body, .{}) catch
+                return AuthError.AuthFlowFailed;
+            const v = parsed.value;
+            if (http.jsonStringAtPath(v, "error")) |e| {
+                if (std.mem.eql(u8, e, "authorization_pending") or std.mem.eql(u8, e, "slow_down"))
+                    return .pending;
+                return AuthError.AuthFlowFailed;
+            }
+            const tokens = parseOAuthTokens(v) orelse return AuthError.AuthFlowFailed;
+            return .{ .done = tokens };
+        },
+        .codex => {
+            const poll_url = oauth.device_poll_url orelse return AuthError.AuthFlowFailed;
+            const enc = try encodeBody(arena, .json, &.{
+                .{ "device_auth_id", da.device_auth_id orelse return AuthError.AuthFlowFailed },
+                .{ "user_code", da.user_code },
+            });
+            const resp = try http.request(arena, client, .POST, poll_url, .{
+                .headers = headers,
+                .body = enc.body,
+                .content_type = enc.content_type,
+            });
+            // 403/404 => still pending (reference behavior).
+            if (resp.status == 403 or resp.status == 404) return .pending;
+            if (!resp.ok()) return AuthError.AuthFlowFailed;
+            const parsed = std.json.parseFromSlice(std.json.Value, arena, resp.body, .{}) catch
+                return AuthError.AuthFlowFailed;
+            const code = http.jsonStringAtPath(parsed.value, "authorization_code") orelse return .pending;
+            const verifier = http.jsonStringAtPath(parsed.value, "code_verifier") orelse return AuthError.AuthFlowFailed;
+            // Exchange the authorization code + server-generated PKCE verifier.
+            const tokens = try exchangeCode(arena, client, oauth, code, verifier, headers);
+            return .{ .done = tokens };
+        },
+    }
+}
+
+/// Codex authorization-code exchange at the token endpoint.
+fn exchangeCode(
+    arena: std.mem.Allocator,
+    client: *std.http.Client,
+    oauth: OAuthDeviceAuth,
+    code: []const u8,
+    verifier: []const u8,
+    headers: []const Header,
+) !OAuthTokens {
+    const redirect = oauth.redirect_uri orelse "https://auth.openai.com/deviceauth/callback";
+    const enc = try encodeBody(arena, .form, &.{
+        .{ "grant_type", "authorization_code" },
+        .{ "client_id", oauth.client_id },
+        .{ "code", code },
+        .{ "code_verifier", verifier },
+        .{ "redirect_uri", redirect },
+    });
+    const resp = try http.request(arena, client, .POST, oauth.token_url, .{
+        .headers = headers,
+        .body = enc.body,
+        .content_type = enc.content_type,
+    });
+    if (!resp.ok()) return AuthError.AuthFlowFailed;
+    const parsed = std.json.parseFromSlice(std.json.Value, arena, resp.body, .{}) catch
+        return AuthError.AuthFlowFailed;
+    return parseOAuthTokens(parsed.value) orelse AuthError.AuthFlowFailed;
+}
+
+/// Refresh an access token. Allocates into `arena`.
+pub fn refreshTokens(
+    arena: std.mem.Allocator,
+    client: *std.http.Client,
+    oauth: OAuthDeviceAuth,
+    refresh_token: []const u8,
+    headers: []const Header,
+) !OAuthTokens {
+    const enc = try encodeBody(arena, .form, &.{
+        .{ "grant_type", "refresh_token" },
+        .{ "refresh_token", refresh_token },
+        .{ "client_id", oauth.client_id },
+    });
+    const resp = try http.request(arena, client, .POST, oauth.token_url, .{
+        .headers = headers,
+        .body = enc.body,
+        .content_type = enc.content_type,
+    });
+    if (!resp.ok()) return AuthError.AuthFlowFailed;
+    const parsed = std.json.parseFromSlice(std.json.Value, arena, resp.body, .{}) catch
+        return AuthError.AuthFlowFailed;
+    return parseOAuthTokens(parsed.value) orelse AuthError.AuthFlowFailed;
+}
+
+/// Pluck an `OAuthTokens` out of a parsed token response. Null if no
+/// `access_token`. Borrows from the parsed value.
+fn parseOAuthTokens(v: std.json.Value) ?OAuthTokens {
+    const access = http.jsonStringAtPath(v, "access_token") orelse return null;
+    return .{
+        .access_token = access,
+        .refresh_token = http.jsonStringAtPath(v, "refresh_token"),
+        .id_token = http.jsonStringAtPath(v, "id_token"),
+        .expires_in = http.jsonIntAtPath(v, "expires_in"),
+    };
+}
+
+/// Run the full interactive device login: request a code, present it, then
+/// poll until authorized (or the deadline passes). Allocates the resulting
+/// tokens into `arena`.
+pub fn login(
+    arena: std.mem.Allocator,
+    io: Io,
+    client: *std.http.Client,
+    oauth: OAuthDeviceAuth,
+    presenter: Presenter,
+    headers: []const Header,
+) !OAuthTokens {
+    const da = try requestDeviceAuth(arena, client, oauth, headers);
+    presenter.deviceCode(.{
+        .verification_uri = da.verification_uri,
+        .user_code = da.user_code,
+        .expires_in = da.expires_in,
+    });
+    presenter.status("waiting for authorization…");
+
+    const start_ns = std.Io.Clock.now(.real, io).nanoseconds;
+    const deadline_ns = start_ns + 15 * std.time.ns_per_min;
+    while (true) {
+        io.sleep(.fromSeconds(@intCast(da.interval_secs)), .real) catch {};
+        const result = try pollOnce(arena, client, oauth, da, headers);
+        switch (result) {
+            .pending => {},
+            .done => |toks| return toks,
+        }
+        if (std.Io.Clock.now(.real, io).nanoseconds >= deadline_ns) return AuthError.AuthLoginTimeout;
+    }
+}
+
+// ===========================================================================
+// Secondary token exchange (Copilot)
+// ===========================================================================
+
+/// Run the configured exchange (Copilot's `/v2/token`) using `bearer` as the
+/// request bearer. Returns the exchanged token + optional expiry/base_url,
+/// allocated into `arena`.
+pub fn runExchange(
+    arena: std.mem.Allocator,
+    client: *std.http.Client,
+    exchange: ExchangeConfig,
+    bearer: []const u8,
+    headers: []const Header,
+) !TokenSet.ExchangeToken {
+    const auth_value = try std.fmt.allocPrint(arena, "Bearer {s}", .{bearer});
+    var hdrs: std.ArrayList(Header) = .empty;
+    try hdrs.append(arena, .{ .name = "authorization", .value = auth_value });
+    for (headers) |h| try hdrs.append(arena, h);
+
+    const method: http.Method = if (std.ascii.eqlIgnoreCase(exchange.method, "POST")) .POST else .GET;
+    const resp = try http.request(arena, client, method, exchange.url, .{ .headers = hdrs.items });
+    if (!resp.ok()) return AuthError.AuthFlowFailed;
+
+    const parsed = std.json.parseFromSlice(std.json.Value, arena, resp.body, .{}) catch
+        return AuthError.AuthFlowFailed;
+    const v = parsed.value;
+    const token = http.jsonStringAtPath(v, exchange.token_json_path) orelse return AuthError.AuthFlowFailed;
+    return .{
+        .token = token,
+        .expires_at = if (exchange.expires_at_json_path) |p| http.jsonIntAtPath(v, p) else null,
+        .base_url = if (exchange.base_url_json_path) |p| http.jsonStringAtPath(v, p) else null,
+    };
+}
+
+// ===========================================================================
+// JWT + credential building (pure)
+// ===========================================================================
+
+/// Decode a JWT's payload (the middle segment) into a parsed JSON value.
+/// Returns null if the token is malformed. Caller owns the result.
+pub fn decodeJwtPayload(allocator: std.mem.Allocator, token: []const u8) ?std.json.Parsed(std.json.Value) {
+    var it = std.mem.splitScalar(u8, token, '.');
+    _ = it.next() orelse return null; // header
+    const payload_b64 = it.next() orelse return null;
+    if (it.next() == null) return null; // require a signature segment
+
+    const dec = std.base64.url_safe_no_pad.Decoder;
+    const n = dec.calcSizeForSlice(payload_b64) catch return null;
+    const buf = allocator.alloc(u8, n) catch return null;
+    defer allocator.free(buf);
+    dec.decode(buf, payload_b64) catch return null;
+    return std.json.parseFromSlice(std.json.Value, allocator, buf, .{}) catch null;
+}
+
+/// The `exp` (unix-seconds expiry) claim of a JWT access/id token, or null.
+pub fn parseJwtExp(allocator: std.mem.Allocator, token: []const u8) ?i64 {
+    var parsed = decodeJwtPayload(allocator, token) orelse return null;
+    defer parsed.deinit();
+    return http.jsonIntAtPath(parsed.value, "exp");
+}
+
+/// Extract the ChatGPT account id from an id_token. `claim_path` is the JWT
+/// claim holding the auth object (e.g. `https://api.openai.com/auth`); the
+/// account id is its `chatgpt_account_id` field. Returns an owned copy.
+pub fn extractAccountId(
+    allocator: std.mem.Allocator,
+    id_token: []const u8,
+    claim_path: []const u8,
+) ?[]u8 {
+    var parsed = decodeJwtPayload(allocator, id_token) orelse return null;
+    defer parsed.deinit();
+    // `claim_path` is a single literal claim key (e.g.
+    // `https://api.openai.com/auth`) that itself contains dots, so it is
+    // looked up directly rather than walked as a dotted path.
+    const claim = switch (parsed.value) {
+        .object => |o| o.get(claim_path) orelse return null,
+        else => return null,
+    };
+    const account = switch (claim) {
+        .object => |o| o.get("chatgpt_account_id") orelse return null,
+        else => return null,
+    };
+    return switch (account) {
+        .string => |s| allocator.dupe(u8, s) catch null,
+        else => null,
+    };
+}
+
+/// True when an access token is missing or within `margin` seconds of expiry.
+pub fn needsRefresh(ts: TokenSet, now_unix: i64, margin: i64) bool {
+    if (ts.access_token == null) return false; // nothing to refresh (login path)
+    const exp = ts.expires_at orelse return false; // no intrinsic expiry (e.g. ghu_)
+    return exp - now_unix <= margin;
+}
+
+/// True when an exchange is configured but the stored exchange token is
+/// missing or within `margin` seconds of expiry.
+pub fn needsExchange(oauth: OAuthDeviceAuth, ts: TokenSet, now_unix: i64, margin: i64) bool {
+    if (oauth.exchange == null) return false;
+    const ex = ts.exchange orelse return true;
+    const exp = ex.expires_at orelse return false; // no expiry => assume durable
+    return exp - now_unix <= margin;
+}
+
+/// Build the request-ready credential from a (refreshed/exchanged) token set.
+/// The exchanged token wins over the access token; a codex account id becomes
+/// a `chatgpt-account-id` header. All strings are duped into `arena`.
+pub fn buildCredential(
+    arena: std.mem.Allocator,
+    oauth: OAuthDeviceAuth,
+    ts: TokenSet,
+) !ResolvedCredential {
+    var base_url_override: ?[]const u8 = null;
+    const secret: []const u8 = blk: {
+        if (ts.exchange) |ex| {
+            if (ex.base_url) |bu| base_url_override = try arena.dupe(u8, bu);
+            break :blk ex.token;
+        }
+        break :blk ts.access_token orelse return AuthError.AuthUnresolved;
+    };
+
+    var headers: std.ArrayList(Header) = .empty;
+    if (oauth.account_id_jwt_claim != null) {
+        if (ts.account_id) |aid| {
+            try headers.append(arena, .{
+                .name = "chatgpt-account-id",
+                .value = try arena.dupe(u8, aid),
+            });
+        }
+    }
+
+    return .{
+        .api_key = try arena.dupe(u8, secret),
+        .base_url_override = base_url_override,
+        .extra_headers = try headers.toOwnedSlice(arena),
+    };
+}
+
+/// Assemble a persisted `TokenSet` from a fresh OAuth token response.
+/// `expires_at` is derived from `expires_in` (preferred) or the access
+/// token's JWT `exp`. A codex account id is extracted when configured. All
+/// strings are duped into `arena`.
+pub fn tokensToTokenSet(
+    arena: std.mem.Allocator,
+    oauth: OAuthDeviceAuth,
+    tokens: OAuthTokens,
+    now_unix: i64,
+) !TokenSet {
+    const access = try arena.dupe(u8, tokens.access_token);
+    const expires_at: ?i64 = blk: {
+        if (tokens.expires_in) |e| break :blk now_unix + e;
+        break :blk parseJwtExp(arena, access);
+    };
+    var account_id: ?[]const u8 = null;
+    if (oauth.account_id_jwt_claim) |claim| {
+        if (tokens.id_token) |idt| {
+            account_id = extractAccountId(arena, idt, claim);
+        }
+    }
+    return .{
+        .type = "oauth_device",
+        .access_token = access,
+        .refresh_token = if (tokens.refresh_token) |r| try arena.dupe(u8, r) else null,
+        .id_token = if (tokens.id_token) |i| try arena.dupe(u8, i) else null,
+        .expires_at = expires_at,
+        .account_id = account_id,
+    };
+}
+
+const t = std.testing;
+
+test "token storage: save, load, delete round-trip" {
+    const io = t.io;
+    var tmp = t.tmpDir(.{});
+    defer tmp.cleanup();
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const n = try tmp.dir.realPath(io, &path_buf);
+    const auth_dir = try std.fmt.allocPrint(t.allocator, "{s}/auth", .{path_buf[0..n]});
+    defer t.allocator.free(auth_dir);
+
+    // Absent => null.
+    try t.expect((try loadTokenSet(t.allocator, io, auth_dir, "ghost")) == null);
+
+    const ts: TokenSet = .{
+        .type = "oauth_device",
+        .access_token = "ghu_abc",
+        .refresh_token = "rt_xyz",
+        .expires_at = 1700000000,
+        .exchange = .{ .token = "tkn", .expires_at = 1700001800, .base_url = "https://api.x" },
+    };
+    try saveTokenSet(t.allocator, io, auth_dir, "github_copilot", ts);
+
+    var loaded = (try loadTokenSet(t.allocator, io, auth_dir, "github_copilot")).?;
+    defer loaded.deinit();
+    try t.expectEqualStrings("ghu_abc", loaded.value.access_token.?);
+    try t.expectEqualStrings("rt_xyz", loaded.value.refresh_token.?);
+    try t.expectEqual(@as(?i64, 1700000000), loaded.value.expires_at);
+    try t.expect(loaded.value.id_token == null);
+    try t.expectEqualStrings("tkn", loaded.value.exchange.?.token);
+    try t.expectEqualStrings("https://api.x", loaded.value.exchange.?.base_url.?);
+
+    try t.expect(try deleteTokenSet(t.allocator, io, auth_dir, "github_copilot"));
+    try t.expect(!try deleteTokenSet(t.allocator, io, auth_dir, "github_copilot"));
+    try t.expect((try loadTokenSet(t.allocator, io, auth_dir, "github_copilot")) == null);
+}
+
+test "AuthConfig: api_key variant" {
+    const a: AuthConfig = .{ .api_key = .{ .key = "sk-resolved" } };
+    try t.expectEqual(AuthType.api_key, @as(AuthType, a));
+    try t.expectEqualStrings("sk-resolved", a.api_key.key.?);
+}
+
+test "AuthConfig: oauth_device defaults" {
+    const a: AuthConfig = .{ .oauth_device = .{
+        .client_id = "Iv1.x",
+        .device_code_url = "https://github.com/login/device/code",
+        .token_url = "https://github.com/login/oauth/access_token",
+    } };
+    try t.expectEqual(DeviceDialect.token, a.oauth_device.dialect);
+    try t.expectEqual(TokenRequestFormat.form, a.oauth_device.token_request_format);
+    try t.expect(a.oauth_device.exchange == null);
+}
+
+test "TokenSet: defaults" {
+    const ts: TokenSet = .{};
+    try t.expectEqualStrings("oauth_device", ts.type);
+    try t.expect(ts.access_token == null);
+    try t.expect(ts.exchange == null);
+}
+
+/// Build a `header.payload.sig` JWT whose payload is `payload_json`,
+/// base64url-no-pad encoded. Allocated in `arena`.
+fn makeJwt(arena: std.mem.Allocator, payload_json: []const u8) ![]u8 {
+    const enc = std.base64.url_safe_no_pad.Encoder;
+    const header = "{\"alg\":\"none\"}";
+    var hbuf: [64]u8 = undefined;
+    const h = enc.encode(&hbuf, header);
+    const pbuf = try arena.alloc(u8, enc.calcSize(payload_json.len));
+    const p = enc.encode(pbuf, payload_json);
+    return std.fmt.allocPrint(arena, "{s}.{s}.sig", .{ h, p });
+}
+
+test "parseJwtExp + extractAccountId" {
+    var aa = std.heap.ArenaAllocator.init(t.allocator);
+    defer aa.deinit();
+    const a = aa.allocator();
+    const jwt = try makeJwt(a,
+        \\{"exp":1700000000,"https://api.openai.com/auth":{"chatgpt_account_id":"acct_123"}}
+    );
+    try t.expectEqual(@as(?i64, 1700000000), parseJwtExp(a, jwt));
+    const aid = extractAccountId(a, jwt, "https://api.openai.com/auth").?;
+    try t.expectEqualStrings("acct_123", aid);
+    // Wrong claim path => null.
+    try t.expect(extractAccountId(a, jwt, "nope") == null);
+    // Malformed token => null.
+    try t.expect(parseJwtExp(a, "not-a-jwt") == null);
+}
+
+test "parseOAuthTokens: required access_token, optional rest" {
+    var parsed = try std.json.parseFromSlice(std.json.Value, t.allocator,
+        \\{"access_token":"at","refresh_token":"rt","id_token":"it","expires_in":1800}
+    , .{});
+    defer parsed.deinit();
+    const toks = parseOAuthTokens(parsed.value).?;
+    try t.expectEqualStrings("at", toks.access_token);
+    try t.expectEqualStrings("rt", toks.refresh_token.?);
+    try t.expectEqual(@as(?i64, 1800), toks.expires_in);
+
+    var p2 = try std.json.parseFromSlice(std.json.Value, t.allocator, "{\"error\":\"x\"}", .{});
+    defer p2.deinit();
+    try t.expect(parseOAuthTokens(p2.value) == null);
+}
+
+test "formEncode: percent-encodes reserved characters" {
+    var aa = std.heap.ArenaAllocator.init(t.allocator);
+    defer aa.deinit();
+    const body = try formEncode(aa.allocator(), &.{
+        .{ "grant_type", "urn:ietf:params:oauth:grant-type:device_code" },
+        .{ "client_id", "Iv1.abc" },
+    });
+    try t.expectEqualStrings(
+        "grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Adevice_code&client_id=Iv1.abc",
+        body,
+    );
+}
+
+test "needsRefresh / needsExchange predicates" {
+    const now: i64 = 1000;
+    // Durable token (no expiry) never needs refresh.
+    try t.expect(!needsRefresh(.{ .access_token = "x" }, now, 120));
+    // Within margin => refresh.
+    try t.expect(needsRefresh(.{ .access_token = "x", .expires_at = 1100 }, now, 120));
+    // Comfortably fresh => no.
+    try t.expect(!needsRefresh(.{ .access_token = "x", .expires_at = 5000 }, now, 120));
+
+    const oauth: OAuthDeviceAuth = .{
+        .client_id = "c",
+        .device_code_url = "u",
+        .token_url = "u",
+        .exchange = .{ .url = "https://x" },
+    };
+    // Exchange configured but none stored => needs exchange.
+    try t.expect(needsExchange(oauth, .{ .access_token = "x" }, now, 120));
+    // Stored exchange near expiry => needs exchange.
+    try t.expect(needsExchange(oauth, .{ .exchange = .{ .token = "t", .expires_at = 1050 } }, now, 120));
+    // Fresh exchange => no.
+    try t.expect(!needsExchange(oauth, .{ .exchange = .{ .token = "t", .expires_at = 9000 } }, now, 120));
+    // No exchange configured => never.
+    const no_ex: OAuthDeviceAuth = .{ .client_id = "c", .device_code_url = "u", .token_url = "u" };
+    try t.expect(!needsExchange(no_ex, .{ .access_token = "x" }, now, 120));
+}
+
+test "buildCredential: copilot exchange token + dynamic base_url" {
+    var aa = std.heap.ArenaAllocator.init(t.allocator);
+    defer aa.deinit();
+    const oauth: OAuthDeviceAuth = .{
+        .client_id = "c",
+        .device_code_url = "u",
+        .token_url = "u",
+        .exchange = .{ .url = "https://x" },
+    };
+    const ts: TokenSet = .{
+        .access_token = "ghu_durable",
+        .exchange = .{ .token = "tid", .base_url = "https://api.individual.githubcopilot.com" },
+    };
+    const cred = try buildCredential(aa.allocator(), oauth, ts);
+    try t.expectEqualStrings("tid", cred.api_key);
+    try t.expectEqualStrings("https://api.individual.githubcopilot.com", cred.base_url_override.?);
+    try t.expectEqual(@as(usize, 0), cred.extra_headers.len);
+}
+
+test "buildCredential: codex access token + account-id header" {
+    var aa = std.heap.ArenaAllocator.init(t.allocator);
+    defer aa.deinit();
+    const oauth: OAuthDeviceAuth = .{
+        .dialect = .codex,
+        .client_id = "c",
+        .device_code_url = "u",
+        .token_url = "u",
+        .device_poll_url = "p",
+        .account_id_jwt_claim = "https://api.openai.com/auth",
+    };
+    const ts: TokenSet = .{ .access_token = "jwt-access", .account_id = "acct_9" };
+    const cred = try buildCredential(aa.allocator(), oauth, ts);
+    try t.expectEqualStrings("jwt-access", cred.api_key);
+    try t.expect(cred.base_url_override == null);
+    try t.expectEqual(@as(usize, 1), cred.extra_headers.len);
+    try t.expectEqualStrings("chatgpt-account-id", cred.extra_headers[0].name);
+    try t.expectEqualStrings("acct_9", cred.extra_headers[0].value);
+}
+
+test "tokensToTokenSet: derives expires_at from expires_in and extracts account_id" {
+    var aa = std.heap.ArenaAllocator.init(t.allocator);
+    defer aa.deinit();
+    const a = aa.allocator();
+    const idt = try makeJwt(a,
+        \\{"https://api.openai.com/auth":{"chatgpt_account_id":"acct_77"}}
+    );
+    const oauth: OAuthDeviceAuth = .{
+        .dialect = .codex,
+        .client_id = "c",
+        .device_code_url = "u",
+        .token_url = "u",
+        .device_poll_url = "p",
+        .account_id_jwt_claim = "https://api.openai.com/auth",
+    };
+    const ts = try tokensToTokenSet(a, oauth, .{
+        .access_token = "at",
+        .refresh_token = "rt",
+        .id_token = idt,
+        .expires_in = 1800,
+    }, 1000);
+    try t.expectEqual(@as(?i64, 2800), ts.expires_at);
+    try t.expectEqualStrings("rt", ts.refresh_token.?);
+    try t.expectEqualStrings("acct_77", ts.account_id.?);
+}