diff --git a/src/atomic.zig b/src/atomic.zig
new file mode 100644
index 0000000..378f835
--- /dev/null
+++ b/src/atomic.zig
@@ -0,0 +1,136 @@
+//! Atomic filesystem writes.
+//!
+//! `writeFileAtomic` writes to `<path>.tmp`, fsyncs, and renames to
+//! `<path>`. Crash-safe replacement for `createFile + writeAll + close`:
+//! if the process dies mid-write, the destination file is left at its
+//! prior contents (or absent) rather than truncated or half-written.
+//!
+//! Used by the snapshot writer so a ctrl-C or kernel panic mid-run
+//! can't produce a corrupt `history/<date>-portfolio.srf`.
+
+const std = @import("std");
+
+/// Suffix appended to the temp file during atomic writes. Exposed so
+/// callers that want to sweep orphan temp files (e.g. from a previous
+/// crash) know what to look for.
+pub const tmp_suffix = ".tmp";
+
+/// Write `bytes` to `path` atomically.
+///
+/// Strategy:
+/// 1. Write to `<path>.tmp` (truncating any previous tmp file).
+/// 2. `fsync` the tmp file so the data is durable before we rename.
+/// 3. Rename tmp -> path (atomic on POSIX when src/dst are on the same
+///    filesystem, which is guaranteed here because both are the literal
+///    path plus `.tmp`).
+///
+/// On any error the tmp file is best-effort removed so we don't leave
+/// clutter behind. The caller's `path` is unchanged unless the final
+/// rename succeeds.
+///
+/// The allocator is used for a short-lived temp-path buffer
+/// (`path.len + tmp_suffix.len` bytes) and freed before return.
+pub fn writeFileAtomic(
+    allocator: std.mem.Allocator,
+    path: []const u8,
+    bytes: []const u8,
+) !void {
+    const tmp_path = try std.fmt.allocPrint(allocator, "{s}{s}", .{ path, tmp_suffix });
+    defer allocator.free(tmp_path);
+
+    {
+        var tmp_file = try std.fs.cwd().createFile(tmp_path, .{
+            .truncate = true,
+            .exclusive = false,
+        });
+        errdefer {
+            tmp_file.close();
+            std.fs.cwd().deleteFile(tmp_path) catch {};
+        }
+
+        try tmp_file.writeAll(bytes);
+        // fsync so the kernel flushes data to disk before the rename
+        // appears. Without this, a crash between rename() and the data
+        // hitting disk could leave an empty-but-present file at `path`.
+        try tmp_file.sync();
+        tmp_file.close();
+    }
+
+    std.fs.cwd().rename(tmp_path, path) catch |err| {
+        std.fs.cwd().deleteFile(tmp_path) catch {};
+        return err;
+    };
+}
+
+// ── Tests ────────────────────────────────────────────────────
+
+test "writeFileAtomic creates new file" {
+    var tmp_dir = std.testing.tmpDir(.{});
+    defer tmp_dir.cleanup();
+
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_path = try tmp_dir.dir.realpath(".", &path_buf);
+    const file_path = try std.fs.path.join(std.testing.allocator, &.{ dir_path, "atomic_new.txt" });
+    defer std.testing.allocator.free(file_path);
+
+    try writeFileAtomic(std.testing.allocator, file_path, "hello world\n");
+
+    const contents = try std.fs.cwd().readFileAlloc(std.testing.allocator, file_path, 4096);
+    defer std.testing.allocator.free(contents);
+    try std.testing.expectEqualStrings("hello world\n", contents);
+
+    // Tmp file should have been consumed by rename.
+    const tmp_path = try std.fmt.allocPrint(std.testing.allocator, "{s}{s}", .{ file_path, tmp_suffix });
+    defer std.testing.allocator.free(tmp_path);
+    try std.testing.expectError(error.FileNotFound, std.fs.cwd().access(tmp_path, .{}));
+
+    // Clean up for the next test run.
+    std.fs.cwd().deleteFile(file_path) catch {};
+}
+
+test "writeFileAtomic overwrites existing file" {
+    var tmp_dir = std.testing.tmpDir(.{});
+    defer tmp_dir.cleanup();
+
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_path = try tmp_dir.dir.realpath(".", &path_buf);
+    const file_path = try std.fs.path.join(std.testing.allocator, &.{ dir_path, "atomic_over.txt" });
+    defer std.testing.allocator.free(file_path);
+
+    // Seed with old content.
+    {
+        var f = try std.fs.cwd().createFile(file_path, .{});
+        try f.writeAll("old contents");
+        f.close();
+    }
+
+    try writeFileAtomic(std.testing.allocator, file_path, "new contents");
+
+    const contents = try std.fs.cwd().readFileAlloc(std.testing.allocator, file_path, 4096);
+    defer std.testing.allocator.free(contents);
+    try std.testing.expectEqualStrings("new contents", contents);
+
+    std.fs.cwd().deleteFile(file_path) catch {};
+}
+
+test "writeFileAtomic: missing parent directory surfaces FileNotFound" {
+    // Point at a path whose parent directory doesn't exist. The tmp dir
+    // itself exists (so the filesystem is fine), but the "missing"
+    // subdirectory does not — createFile on the .tmp file must fail
+    // with FileNotFound regardless of platform.
+    var tmp_dir = std.testing.tmpDir(.{});
+    defer tmp_dir.cleanup();
+
+    var path_buf: [std.fs.max_path_bytes]u8 = undefined;
+    const dir_path = try tmp_dir.dir.realpath(".", &path_buf);
+    const bad_path = try std.fs.path.join(
+        std.testing.allocator,
+        &.{ dir_path, "missing", "file.txt" },
+    );
+    defer std.testing.allocator.free(bad_path);
+
+    try std.testing.expectError(
+        error.FileNotFound,
+        writeFileAtomic(std.testing.allocator, bad_path, "x"),
+    );
+}
diff --git a/src/commands/snapshot.zig b/src/commands/snapshot.zig
new file mode 100644
index 0000000..a640afe
--- /dev/null
+++ b/src/commands/snapshot.zig
@@ -0,0 +1,899 @@
+//! `zfin snapshot` — write a daily portfolio snapshot to `history/`.
+//!
+//! Flow:
+//!   1. Locate portfolio.srf via `config.resolveUserFile` (or -p).
+//!   2. Derive `history/` dir as `dirname(portfolio.srf)/history/`.
+//!   3. Load portfolio + prices (via `cli.loadPortfolioPrices`, TTL-driven).
+//!   4. Compute `as_of_date` from cached candle dates of held non-MM
+//!      stock symbols.
+//!   5. If `history/<as_of_date>-portfolio.srf` already exists and
+//!      --force wasn't passed, skip (exit 0, stderr message).
+//!   6. Build the snapshot records and write them atomically.
+//!
+//! The output format is discriminated SRF: every record starts with
+//! `kind::<meta|total|tax_type|account|lot>`. Readers demux on that
+//! field. See finance/README.md for the full schema.
+
+const std = @import("std");
+const srf = @import("srf");
+const zfin = @import("../root.zig");
+const cli = @import("common.zig");
+const atomic = @import("../atomic.zig");
+const version = @import("../version.zig");
+const portfolio_mod = @import("../models/portfolio.zig");
+const Date = @import("../models/date.zig").Date;
+
+pub const SnapshotError = error{
+    PortfolioEmpty,
+    WriteFailed,
+};
+
+// ── Entry point ──────────────────────────────────────────────
+
+/// Run the snapshot command.
+///
+/// `args` is the slice after `zfin snapshot`. Accepted flags:
+///   --force           overwrite existing snapshot for as_of_date
+///   --out <path>      override output path (skips the default derivation)
+///   --dry-run         compute + print, do not write
+///
+/// Exit semantics:
+///   0 on success (including duplicate-skip)
+///   non-zero on any error
+pub fn run(
+    allocator: std.mem.Allocator,
+    svc: *zfin.DataService,
+    portfolio_path: []const u8,
+    args: []const []const u8,
+    color: bool,
+    out: *std.Io.Writer,
+) !void {
+    // Parse flags.
+    var force = false;
+    var dry_run = false;
+    var out_override: ?[]const u8 = null;
+    var i: usize = 0;
+    while (i < args.len) : (i += 1) {
+        const a = args[i];
+        if (std.mem.eql(u8, a, "--force")) {
+            force = true;
+        } else if (std.mem.eql(u8, a, "--dry-run")) {
+            dry_run = true;
+        } else if (std.mem.eql(u8, a, "--out")) {
+            i += 1;
+            if (i >= args.len) {
+                try cli.stderrPrint("Error: --out requires a path argument\n");
+                return error.UnexpectedArg;
+            }
+            out_override = args[i];
+        } else {
+            try cli.stderrPrint("Error: unknown argument to 'snapshot': ");
+            try cli.stderrPrint(a);
+            try cli.stderrPrint("\n");
+            return error.UnexpectedArg;
+        }
+    }
+
+    // Load portfolio.
+    const pf_data = std.fs.cwd().readFileAlloc(allocator, portfolio_path, 10 * 1024 * 1024) catch |err| {
+        try cli.stderrPrint("Error reading portfolio file: ");
+        try cli.stderrPrint(@errorName(err));
+        try cli.stderrPrint("\n");
+        return err;
+    };
+    defer allocator.free(pf_data);
+
+    var portfolio = zfin.cache.deserializePortfolio(allocator, pf_data) catch {
+        try cli.stderrPrint("Error parsing portfolio file.\n");
+        return error.WriteFailed;
+    };
+    defer portfolio.deinit();
+
+    if (portfolio.lots.len == 0) {
+        try cli.stderrPrint("Portfolio is empty; nothing to snapshot.\n");
+        return SnapshotError.PortfolioEmpty;
+    }
+
+    // Fetch prices via the shared TTL-driven loader.
+    const syms = try portfolio.stockSymbols(allocator);
+    defer allocator.free(syms);
+
+    var prices = std.StringHashMap(f64).init(allocator);
+    defer prices.deinit();
+
+    if (syms.len > 0) {
+        var load_result = cli.loadPortfolioPrices(svc, syms, &.{}, false, color);
+        defer load_result.deinit();
+        var it = load_result.prices.iterator();
+        while (it.next()) |entry| {
+            try prices.put(entry.key_ptr.*, entry.value_ptr.*);
+        }
+    }
+
+    // Manual `price::` overrides from portfolio.srf still win.
+    for (portfolio.lots) |lot| {
+        if (lot.price) |p| {
+            if (!prices.contains(lot.priceSymbol())) {
+                try prices.put(lot.priceSymbol(), p * lot.price_ratio);
+            }
+        }
+    }
+
+    // Compute as_of_date from the cached candle dates of held non-MM
+    // stock symbols. MM symbols are excluded because their quote dates
+    // are often weeks stale (dollar impact is nil, but they'd pollute
+    // the mode calculation).
+    const qdates = try collectQuoteDates(allocator, svc, syms);
+    defer allocator.free(qdates.dates);
+    const as_of = computeAsOfDate(qdates.dates) orelse Date.fromEpoch(std.time.timestamp());
+
+    // Derive output path.
+    var as_of_buf: [10]u8 = undefined;
+    const as_of_str = as_of.format(&as_of_buf);
+
+    const derived_path = if (out_override) |p|
+        try allocator.dupe(u8, p)
+    else
+        try deriveSnapshotPath(allocator, portfolio_path, as_of_str);
+    defer allocator.free(derived_path);
+
+    // Duplicate-skip check.
+    if (!force and !dry_run) {
+        if (std.fs.cwd().access(derived_path, .{})) |_| {
+            var msg_buf: [256]u8 = undefined;
+            const msg = std.fmt.bufPrint(&msg_buf, "snapshot for {s} already exists: {s} (use --force to overwrite)\n", .{ as_of_str, derived_path }) catch "snapshot already exists\n";
+            try cli.stderrPrint(msg);
+            return;
+        } else |_| {}
+    }
+
+    // Build and render the snapshot.
+    var snap = try buildSnapshot(allocator, &portfolio, portfolio_path, svc, prices, syms, as_of, qdates);
+    defer snap.deinit(allocator);
+
+    const rendered = try renderSnapshot(allocator, snap);
+    defer allocator.free(rendered);
+
+    if (dry_run) {
+        try out.writeAll(rendered);
+        return;
+    }
+
+    // Ensure history/ directory exists.
+    if (std.fs.path.dirname(derived_path)) |dir| {
+        std.fs.cwd().makePath(dir) catch |err| switch (err) {
+            error.PathAlreadyExists => {},
+            else => {
+                try cli.stderrPrint("Error creating history directory: ");
+                try cli.stderrPrint(@errorName(err));
+                try cli.stderrPrint("\n");
+                return err;
+            },
+        };
+    }
+
+    atomic.writeFileAtomic(allocator, derived_path, rendered) catch |err| {
+        try cli.stderrPrint("Error writing snapshot: ");
+        try cli.stderrPrint(@errorName(err));
+        try cli.stderrPrint("\n");
+        return err;
+    };
+
+    try out.print("snapshot written: {s}\n", .{derived_path});
+}
+
+// ── Path helpers ─────────────────────────────────────────────
+
+/// Derive `<dir(portfolio_path)>/history/<as_of>-portfolio.srf`.
+/// Caller owns returned memory.
+pub fn deriveSnapshotPath(
+    allocator: std.mem.Allocator,
+    portfolio_path: []const u8,
+    as_of_str: []const u8,
+) ![]const u8 {
+    const dir = std.fs.path.dirname(portfolio_path) orelse ".";
+    const filename = try std.fmt.allocPrint(allocator, "{s}-portfolio.srf", .{as_of_str});
+    defer allocator.free(filename);
+    return std.fs.path.join(allocator, &.{ dir, "history", filename });
+}
+
+// ── Quote-date / as_of_date helpers ──────────────────────────
+
+/// Per-symbol quote-date info gathered from the candle cache.
+pub const QuoteInfo = struct {
+    symbol: []const u8, // borrowed from caller
+    /// Most recent candle date in cache, if any candles exist.
+    last_date: ?Date,
+    /// True when the symbol is classified money-market (excluded from
+    /// as_of_date computation because MM candles are often stale but
+    /// dollar-impact is nil).
+    is_money_market: bool,
+};
+
+pub const QuoteDates = struct {
+    /// Per-symbol info (same order as input `symbols`).
+    dates: []QuoteInfo,
+};
+
+/// Gather quote-date info for each symbol from the cache. Does not
+/// fetch; relies on whatever the cache has. Symbols with no candles at
+/// all get `last_date = null`.
+pub fn collectQuoteDates(
+    allocator: std.mem.Allocator,
+    svc: *zfin.DataService,
+    symbols: []const []const u8,
+) !QuoteDates {
+    var list = try allocator.alloc(QuoteInfo, symbols.len);
+    errdefer allocator.free(list);
+
+    for (symbols, 0..) |sym, idx| {
+        const is_mm = portfolio_mod.isMoneyMarketSymbol(sym);
+        var last_date: ?Date = null;
+        if (svc.getCachedCandles(sym)) |cs| {
+            defer allocator.free(cs);
+            if (cs.len > 0) last_date = cs[cs.len - 1].date;
+        }
+        list[idx] = .{ .symbol = sym, .last_date = last_date, .is_money_market = is_mm };
+    }
+
+    return .{ .dates = list };
+}
+
+/// Compute the snapshot's `as_of_date` from per-symbol quote info.
+///
+/// Rule: take the **mode** of `last_date` across non-MM symbols with a
+/// known date; break ties by picking the maximum. If no non-MM symbol
+/// has a date (portfolio is all cash/MM, or totally uncached), return
+/// null and the caller falls back to the capture date.
+pub fn computeAsOfDate(infos: []const QuoteInfo) ?Date {
+    // Two-pass mode: count occurrences, then pick max-count/max-date.
+    // With typical portfolios (~30 symbols) O(n²) is fine.
+    var best: ?Date = null;
+    var best_count: usize = 0;
+
+    for (infos) |a| {
+        if (a.is_money_market) continue;
+        const a_date = a.last_date orelse continue;
+
+        var count: usize = 0;
+        for (infos) |b| {
+            if (b.is_money_market) continue;
+            const b_date = b.last_date orelse continue;
+            if (a_date.eql(b_date)) count += 1;
+        }
+
+        if (count > best_count or
+            (count == best_count and best != null and best.?.lessThan(a_date)))
+        {
+            best = a_date;
+            best_count = count;
+        }
+    }
+
+    return best;
+}
+
+/// Return (min, max) of non-MM symbol quote dates, for metadata.
+/// Returns null if no non-MM symbol has a known date.
+pub fn quoteDateRange(infos: []const QuoteInfo) ?struct { min: Date, max: Date } {
+    var min_d: ?Date = null;
+    var max_d: ?Date = null;
+    for (infos) |info| {
+        if (info.is_money_market) continue;
+        const d = info.last_date orelse continue;
+        if (min_d == null or d.lessThan(min_d.?)) min_d = d;
+        if (max_d == null or max_d.?.lessThan(d)) max_d = d;
+    }
+    if (min_d == null) return null;
+    return .{ .min = min_d.?, .max = max_d.? };
+}
+
+// ── Snapshot records ─────────────────────────────────────────
+//
+// Each record kind below is a plain struct suitable for `srf.fmtFrom`.
+// Field order in the struct declaration IS the on-disk order — srf's
+// `Record.from` iterates `inline for (info.fields)`. The leading `kind`
+// field is the discriminator readers demux on.
+//
+// IMPORTANT: `kind` does NOT have a default value. SRF elides fields
+// whose value matches the declared default (see setField in srf.zig),
+// so a `kind: []const u8 = "meta"` would vanish from the output. Each
+// construction site supplies the tag explicitly.
+//
+// Optional fields default to `null` so they're elided on null values —
+// that's the behavior we want for `price`, `quote_date`, etc.
+
+pub const MetaRow = struct {
+    kind: []const u8,
+    snapshot_version: u32,
+    as_of_date: Date,
+    captured_at: i64,
+    zfin_version: []const u8,
+    quote_date_min: ?Date = null,
+    quote_date_max: ?Date = null,
+    stale_count: usize,
+};
+
+pub const TotalRow = struct {
+    kind: []const u8,
+    scope: []const u8,
+    value: f64,
+};
+
+pub const TaxTypeRow = struct {
+    kind: []const u8,
+    label: []const u8,
+    value: f64,
+};
+
+pub const AccountRow = struct {
+    kind: []const u8,
+    name: []const u8,
+    value: f64,
+};
+
+pub const LotRow = struct {
+    kind: []const u8,
+    symbol: []const u8,
+    lot_symbol: []const u8,
+    account: []const u8,
+    security_type: []const u8,
+    shares: f64,
+    open_price: f64,
+    cost_basis: f64,
+    value: f64,
+    /// Null for non-stock lots (cash/CD/illiquid have no per-share price).
+    price: ?f64 = null,
+    /// Null for non-stock lots.
+    quote_date: ?Date = null,
+    /// Emitted only when true (default is false, which srf skips).
+    quote_stale: bool = false,
+};
+
+pub const Snapshot = struct {
+    meta: MetaRow,
+    totals: []TotalRow,
+    tax_types: []TaxTypeRow,
+    accounts: []AccountRow,
+    lots: []LotRow,
+
+    pub fn deinit(self: *Snapshot, allocator: std.mem.Allocator) void {
+        allocator.free(self.totals);
+        allocator.free(self.tax_types);
+        allocator.free(self.accounts);
+        allocator.free(self.lots);
+    }
+};
+
+/// Build the full snapshot in memory. Does not touch disk.
+fn buildSnapshot(
+    allocator: std.mem.Allocator,
+    portfolio: *zfin.Portfolio,
+    portfolio_path: []const u8,
+    svc: *zfin.DataService,
+    prices: std.StringHashMap(f64),
+    syms: []const []const u8,
+    as_of: Date,
+    qdates: QuoteDates,
+) !Snapshot {
+    // Totals
+    const positions = try portfolio.positions(allocator);
+    defer allocator.free(positions);
+
+    var manual_set = try zfin.valuation.buildFallbackPrices(allocator, portfolio.lots, positions, @constCast(&prices));
+    defer manual_set.deinit();
+
+    var summary = try zfin.valuation.portfolioSummary(allocator, portfolio.*, positions, prices, manual_set);
+    defer summary.deinit(allocator);
+
+    const illiquid = portfolio.totalIlliquid();
+    const net_worth = zfin.valuation.netWorth(portfolio.*, summary);
+
+    var totals = try allocator.alloc(TotalRow, 3);
+    totals[0] = .{ .kind = "total", .scope = "net_worth", .value = net_worth };
+    totals[1] = .{ .kind = "total", .scope = "liquid", .value = summary.total_value };
+    totals[2] = .{ .kind = "total", .scope = "illiquid", .value = illiquid };
+
+    // Analysis (optional — depends on metadata.srf existing). If it
+    // fails we still emit the snapshot with empty tax_type/account
+    // sections rather than failing the whole capture.
+    var tax_types: []TaxTypeRow = &.{};
+    var accounts: []AccountRow = &.{};
+
+    if (runAnalysis(allocator, portfolio, portfolio_path, svc, summary)) |result| {
+        var a = result;
+        defer a.deinit(allocator);
+
+        tax_types = try allocator.alloc(TaxTypeRow, a.tax_type.len);
+        for (a.tax_type, 0..) |t, idx| {
+            tax_types[idx] = .{ .kind = "tax_type", .label = t.label, .value = t.value };
+        }
+        errdefer allocator.free(tax_types);
+
+        accounts = try allocator.alloc(AccountRow, a.account.len);
+        for (a.account, 0..) |acc, idx| {
+            accounts[idx] = .{ .kind = "account", .name = acc.label, .value = acc.value };
+        }
+    } else |_| {
+        // Silent: metadata.srf may legitimately not exist during initial
+        // setup. Header is already emitted; missing-analysis just means
+        // fewer breakdowns in the snapshot.
+    }
+
+    // Per-lot rows (open lots only). Stock lots get current price +
+    // stale flag; non-stock lots get face value.
+    var lots_list = std.ArrayList(LotRow).empty;
+    errdefer lots_list.deinit(allocator);
+
+    var stale_count: usize = 0;
+    _ = syms;
+
+    for (portfolio.lots) |lot| {
+        if (!lot.isOpen()) continue;
+
+        const sec_label = lot.security_type.label();
+        const lot_sym = lot.symbol;
+        const price_sym = lot.priceSymbol();
+        const acct = lot.account orelse "";
+
+        switch (lot.security_type) {
+            .stock => {
+                const raw_price = prices.get(price_sym) orelse lot.open_price;
+                const is_manual = manual_set.contains(price_sym);
+                const effective_price = if (is_manual) raw_price else raw_price * lot.price_ratio;
+                const value = lot.shares * effective_price;
+
+                var quote_date: ?Date = null;
+                for (qdates.dates) |qi| {
+                    if (std.mem.eql(u8, qi.symbol, price_sym)) {
+                        quote_date = qi.last_date;
+                        break;
+                    }
+                }
+                const stale = if (quote_date) |qd| !qd.eql(as_of) else false;
+                if (stale and !portfolio_mod.isMoneyMarketSymbol(price_sym)) stale_count += 1;
+
+                try lots_list.append(allocator, .{
+                    .kind = "lot",
+                    .symbol = price_sym,
+                    .lot_symbol = lot_sym,
+                    .account = acct,
+                    .security_type = sec_label,
+                    .shares = lot.shares,
+                    .open_price = lot.open_price,
+                    .cost_basis = lot.costBasis(),
+                    .price = effective_price,
+                    .value = value,
+                    .quote_date = quote_date,
+                    .quote_stale = stale,
+                });
+            },
+            .cash, .cd, .illiquid => {
+                // `shares` is the face/dollar value for these types.
+                try lots_list.append(allocator, .{
+                    .kind = "lot",
+                    .symbol = lot_sym,
+                    .lot_symbol = lot_sym,
+                    .account = acct,
+                    .security_type = sec_label,
+                    .shares = lot.shares,
+                    .open_price = 0,
+                    .cost_basis = 0,
+                    .value = lot.shares,
+                });
+            },
+            .option => {
+                const opt_value = @abs(lot.shares) * lot.open_price * lot.multiplier;
+                try lots_list.append(allocator, .{
+                    .kind = "lot",
+                    .symbol = lot_sym,
+                    .lot_symbol = lot_sym,
+                    .account = acct,
+                    .security_type = sec_label,
+                    .shares = lot.shares,
+                    .open_price = lot.open_price,
+                    .cost_basis = opt_value,
+                    .value = opt_value,
+                });
+            },
+            .watch => {
+                // Watchlist lots aren't positions — skip.
+            },
+        }
+    }
+
+    const range = quoteDateRange(qdates.dates);
+
+    return .{
+        .meta = .{
+            .kind = "meta",
+            .snapshot_version = 1,
+            .as_of_date = as_of,
+            .captured_at = std.time.timestamp(),
+            .zfin_version = version.version_string,
+            .quote_date_min = if (range) |r| r.min else null,
+            .quote_date_max = if (range) |r| r.max else null,
+            .stale_count = stale_count,
+        },
+        .totals = totals,
+        .tax_types = tax_types,
+        .accounts = accounts,
+        .lots = try lots_list.toOwnedSlice(allocator),
+    };
+}
+
+fn runAnalysis(
+    allocator: std.mem.Allocator,
+    portfolio: *zfin.Portfolio,
+    portfolio_path: []const u8,
+    svc: *zfin.DataService,
+    summary: zfin.valuation.PortfolioSummary,
+) !zfin.analysis.AnalysisResult {
+    const dir_end = if (std.mem.lastIndexOfScalar(u8, portfolio_path, std.fs.path.sep)) |idx| idx + 1 else 0;
+    const meta_path = try std.fmt.allocPrint(allocator, "{s}metadata.srf", .{portfolio_path[0..dir_end]});
+    defer allocator.free(meta_path);
+
+    const meta_data = std.fs.cwd().readFileAlloc(allocator, meta_path, 1024 * 1024) catch return error.NoMetadata;
+    defer allocator.free(meta_data);
+
+    var cm = zfin.classification.parseClassificationFile(allocator, meta_data) catch return error.BadMetadata;
+    defer cm.deinit();
+
+    var acct_map_opt: ?zfin.analysis.AccountMap = svc.loadAccountMap(portfolio_path);
+    defer if (acct_map_opt) |*am| am.deinit();
+
+    return zfin.analysis.analyzePortfolio(
+        allocator,
+        summary.allocations,
+        cm,
+        portfolio.*,
+        summary.total_value,
+        acct_map_opt,
+    );
+}
+
+// ── SRF rendering ────────────────────────────────────────────
+
+/// Render a snapshot to SRF bytes. Caller owns result.
+///
+/// Each section is emitted as a homogeneous record slice via
+/// `srf.fmtFrom`. The first section (meta) carries `emit_directives =
+/// true` so the `#!srfv1` header and `#!created=...` line are written
+/// once at the top; subsequent sections set `emit_directives = false`
+/// to suppress a duplicate header.
+pub fn renderSnapshot(allocator: std.mem.Allocator, snap: Snapshot) ![]const u8 {
+    var aw: std.Io.Writer.Allocating = .init(allocator);
+    errdefer aw.deinit();
+    const w = &aw.writer;
+
+    // Single-element slice so we can route the meta row through the
+    // same `fmtFrom` pipeline as the rest of the sections. This also
+    // puts the `#!created=...` header at the top of the file.
+    const meta_rows: [1]MetaRow = .{snap.meta};
+    try w.print("{f}", .{srf.fmtFrom(MetaRow, allocator, &meta_rows, .{
+        .emit_directives = true,
+        .created = snap.meta.captured_at,
+    })});
+
+    // Subsequent sections: records only (no header).
+    const tail_opts: srf.FormatOptions = .{ .emit_directives = false };
+    try w.print("{f}", .{srf.fmtFrom(TotalRow, allocator, snap.totals, tail_opts)});
+    try w.print("{f}", .{srf.fmtFrom(TaxTypeRow, allocator, snap.tax_types, tail_opts)});
+    try w.print("{f}", .{srf.fmtFrom(AccountRow, allocator, snap.accounts, tail_opts)});
+    try w.print("{f}", .{srf.fmtFrom(LotRow, allocator, snap.lots, tail_opts)});
+
+    return aw.toOwnedSlice();
+}
+
+// ── Tests ────────────────────────────────────────────────────
+
+const testing = std.testing;
+
+test "deriveSnapshotPath: standard layout" {
+    // Build the input portfolio path and expected output from
+    // path-joined components so the test runs on both POSIX and Windows.
+    const portfolio_path = try std.fs.path.join(
+        testing.allocator,
+        &.{ "home", "lobo", "finance", "portfolio.srf" },
+    );
+    defer testing.allocator.free(portfolio_path);
+
+    const expected = try std.fs.path.join(
+        testing.allocator,
+        &.{ "home", "lobo", "finance", "history", "2026-04-20-portfolio.srf" },
+    );
+    defer testing.allocator.free(expected);
+
+    const p = try deriveSnapshotPath(testing.allocator, portfolio_path, "2026-04-20");
+    defer testing.allocator.free(p);
+    try testing.expectEqualStrings(expected, p);
+}
+
+test "deriveSnapshotPath: bare filename (no dir) falls back to cwd" {
+    const expected = try std.fs.path.join(
+        testing.allocator,
+        &.{ ".", "history", "2026-04-20-portfolio.srf" },
+    );
+    defer testing.allocator.free(expected);
+
+    const p = try deriveSnapshotPath(testing.allocator, "portfolio.srf", "2026-04-20");
+    defer testing.allocator.free(p);
+    try testing.expectEqualStrings(expected, p);
+}
+
+test "computeAsOfDate: mode of non-MM dates, ties broken by max" {
+    const d1 = Date.fromYmd(2026, 4, 17);
+    const d2 = Date.fromYmd(2026, 4, 20);
+    const infos = [_]QuoteInfo{
+        .{ .symbol = "VTI", .last_date = d2, .is_money_market = false },
+        .{ .symbol = "AAPL", .last_date = d2, .is_money_market = false },
+        .{ .symbol = "MSFT", .last_date = d1, .is_money_market = false },
+        // Money-market with stale date — must not win the mode.
+        .{ .symbol = "SWVXX", .last_date = Date.fromYmd(2025, 1, 1), .is_money_market = true },
+    };
+    const result = computeAsOfDate(&infos);
+    try testing.expect(result != null);
+    try testing.expect(result.?.eql(d2));
+}
+
+test "computeAsOfDate: ties break toward max date" {
+    const d1 = Date.fromYmd(2026, 4, 17);
+    const d2 = Date.fromYmd(2026, 4, 20);
+    const infos = [_]QuoteInfo{
+        .{ .symbol = "A", .last_date = d1, .is_money_market = false },
+        .{ .symbol = "B", .last_date = d2, .is_money_market = false },
+    };
+    const result = computeAsOfDate(&infos).?;
+    try testing.expect(result.eql(d2));
+}
+
+test "computeAsOfDate: all MM returns null" {
+    const infos = [_]QuoteInfo{
+        .{ .symbol = "SWVXX", .last_date = Date.fromYmd(2026, 4, 20), .is_money_market = true },
+        .{ .symbol = "VMFXX", .last_date = Date.fromYmd(2026, 4, 20), .is_money_market = true },
+    };
+    try testing.expect(computeAsOfDate(&infos) == null);
+}
+
+test "computeAsOfDate: symbols with no cached date are ignored" {
+    const d = Date.fromYmd(2026, 4, 20);
+    const infos = [_]QuoteInfo{
+        .{ .symbol = "VTI", .last_date = d, .is_money_market = false },
+        .{ .symbol = "UNCACHED", .last_date = null, .is_money_market = false },
+    };
+    const result = computeAsOfDate(&infos).?;
+    try testing.expect(result.eql(d));
+}
+
+test "computeAsOfDate: empty input returns null" {
+    try testing.expect(computeAsOfDate(&.{}) == null);
+}
+
+test "quoteDateRange: min and max skip MM symbols" {
+    const d_old = Date.fromYmd(2026, 4, 17);
+    const d_new = Date.fromYmd(2026, 4, 20);
+    const d_ancient = Date.fromYmd(2025, 1, 1);
+    const infos = [_]QuoteInfo{
+        .{ .symbol = "A", .last_date = d_new, .is_money_market = false },
+        .{ .symbol = "B", .last_date = d_old, .is_money_market = false },
+        // MM way older — must be excluded from the range.
+        .{ .symbol = "SWVXX", .last_date = d_ancient, .is_money_market = true },
+    };
+    const r = quoteDateRange(&infos).?;
+    try testing.expect(r.min.eql(d_old));
+    try testing.expect(r.max.eql(d_new));
+}
+
+test "quoteDateRange: returns null when no non-MM data" {
+    const infos = [_]QuoteInfo{
+        .{ .symbol = "SWVXX", .last_date = Date.fromYmd(2026, 4, 20), .is_money_market = true },
+    };
+    try testing.expect(quoteDateRange(&infos) == null);
+}
+
+test "renderSnapshot: minimal snapshot shape" {
+    const totals = [_]TotalRow{
+        .{ .kind = "total", .scope = "net_worth", .value = 1000.0 },
+        .{ .kind = "total", .scope = "liquid", .value = 800.0 },
+        .{ .kind = "total", .scope = "illiquid", .value = 200.0 },
+    };
+    const snap: Snapshot = .{
+        .meta = .{
+            .kind = "meta",
+            .snapshot_version = 1,
+            .as_of_date = Date.fromYmd(2026, 4, 20),
+            .captured_at = 1_745_222_400,
+            .zfin_version = "testver",
+            .stale_count = 0,
+        },
+        .totals = @constCast(&totals),
+        .tax_types = &.{},
+        .accounts = &.{},
+        .lots = &.{},
+    };
+
+    const rendered = try renderSnapshot(testing.allocator, snap);
+    defer testing.allocator.free(rendered);
+
+    // Header + front-matter from the first fmtFrom call.
+    try testing.expect(std.mem.startsWith(u8, rendered, "#!srfv1\n"));
+    try testing.expect(std.mem.indexOf(u8, rendered, "#!created=1745222400") != null);
+
+    // Meta record fields (discriminator, version, date, captured_at).
+    try testing.expect(std.mem.indexOf(u8, rendered, "kind::meta") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered, "as_of_date::2026-04-20") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered, "zfin_version::testver") != null);
+
+    // Totals records use kind::total plus scope+value.
+    try testing.expect(std.mem.indexOf(u8, rendered, "kind::total,scope::net_worth") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered, "kind::total,scope::liquid") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered, "kind::total,scope::illiquid") != null);
+}
+
+test "renderSnapshot: includes quote_date_min/max when present, elided when null" {
+    const snap_with: Snapshot = .{
+        .meta = .{
+            .kind = "meta",
+            .snapshot_version = 1,
+            .as_of_date = Date.fromYmd(2026, 4, 20),
+            .captured_at = 0,
+            .zfin_version = "x",
+            .quote_date_min = Date.fromYmd(2026, 4, 17),
+            .quote_date_max = Date.fromYmd(2026, 4, 20),
+            .stale_count = 2,
+        },
+        .totals = &.{},
+        .tax_types = &.{},
+        .accounts = &.{},
+        .lots = &.{},
+    };
+    const rendered_with = try renderSnapshot(testing.allocator, snap_with);
+    defer testing.allocator.free(rendered_with);
+    try testing.expect(std.mem.indexOf(u8, rendered_with, "quote_date_min::2026-04-17") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered_with, "quote_date_max::2026-04-20") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered_with, "stale_count:num:2") != null);
+
+    // Same structure with nulls — srf elides optional fields matching
+    // their `null` default, so those keys must NOT appear.
+    const snap_without: Snapshot = .{
+        .meta = .{
+            .kind = "meta",
+            .snapshot_version = 1,
+            .as_of_date = Date.fromYmd(2026, 4, 20),
+            .captured_at = 0,
+            .zfin_version = "x",
+            .stale_count = 0,
+        },
+        .totals = &.{},
+        .tax_types = &.{},
+        .accounts = &.{},
+        .lots = &.{},
+    };
+    const rendered_without = try renderSnapshot(testing.allocator, snap_without);
+    defer testing.allocator.free(rendered_without);
+    try testing.expect(std.mem.indexOf(u8, rendered_without, "quote_date_min") == null);
+    try testing.expect(std.mem.indexOf(u8, rendered_without, "quote_date_max") == null);
+}
+
+test "renderSnapshot: lot rendering elides price/quote_date/stale when default" {
+    const lots = [_]LotRow{
+        // Stock lot — all three optional fields populated.
+        .{
+            .kind = "lot",
+            .symbol = "VTI",
+            .lot_symbol = "VTI",
+            .account = "Emil Roth",
+            .security_type = "Stock",
+            .shares = 100,
+            .open_price = 200.0,
+            .cost_basis = 20000.0,
+            .value = 31002.0,
+            .price = 310.02,
+            .quote_date = Date.fromYmd(2026, 4, 17),
+            .quote_stale = true,
+        },
+        // Cash lot — optionals left at default (null / false), so srf
+        // elides them.
+        .{
+            .kind = "lot",
+            .symbol = "Savings",
+            .lot_symbol = "Savings",
+            .account = "Emil Roth",
+            .security_type = "Cash",
+            .shares = 50000,
+            .open_price = 0,
+            .cost_basis = 0,
+            .value = 50000,
+        },
+    };
+    const snap: Snapshot = .{
+        .meta = .{
+            .kind = "meta",
+            .snapshot_version = 1,
+            .as_of_date = Date.fromYmd(2026, 4, 20),
+            .captured_at = 0,
+            .zfin_version = "x",
+            .stale_count = 1,
+        },
+        .totals = &.{},
+        .tax_types = &.{},
+        .accounts = &.{},
+        .lots = @constCast(&lots),
+    };
+    const rendered = try renderSnapshot(testing.allocator, snap);
+    defer testing.allocator.free(rendered);
+
+    // Stock lot line: extract it so we can check in isolation.
+    const vti_start = std.mem.indexOf(u8, rendered, "kind::lot,symbol::VTI").?;
+    const vti_end = std.mem.indexOfScalarPos(u8, rendered, vti_start, '\n').?;
+    const vti_line = rendered[vti_start..vti_end];
+    // All three optional-on-stock fields present.
+    try testing.expect(std.mem.indexOf(u8, vti_line, ",price:num:") != null);
+    try testing.expect(std.mem.indexOf(u8, vti_line, "quote_date::2026-04-17") != null);
+    try testing.expect(std.mem.indexOf(u8, vti_line, "quote_stale") != null);
+
+    // Cash lot line: the three optional fields must be elided because
+    // they match their declared defaults (null, null, false).
+    const cash_start = std.mem.indexOf(u8, rendered, "kind::lot,symbol::Savings").?;
+    const cash_end = std.mem.indexOfScalarPos(u8, rendered, cash_start, '\n').?;
+    const cash_line = rendered[cash_start..cash_end];
+    try testing.expect(std.mem.indexOf(u8, cash_line, ",price:num:") == null);
+    try testing.expect(std.mem.indexOf(u8, cash_line, "quote_date") == null);
+    try testing.expect(std.mem.indexOf(u8, cash_line, "quote_stale") == null);
+}
+
+test "renderSnapshot: tax_type and account rows carry kind discriminator" {
+    const tax = [_]TaxTypeRow{
+        .{ .kind = "tax_type", .label = "Taxable", .value = 5000 },
+        .{ .kind = "tax_type", .label = "Roth (Post-Tax)", .value = 3000 },
+    };
+    const accts = [_]AccountRow{
+        .{ .kind = "account", .name = "Emil Roth", .value = 2500 },
+    };
+    const snap: Snapshot = .{
+        .meta = .{
+            .kind = "meta",
+            .snapshot_version = 1,
+            .as_of_date = Date.fromYmd(2026, 4, 20),
+            .captured_at = 0,
+            .zfin_version = "x",
+            .stale_count = 0,
+        },
+        .totals = &.{},
+        .tax_types = @constCast(&tax),
+        .accounts = @constCast(&accts),
+        .lots = &.{},
+    };
+    const rendered = try renderSnapshot(testing.allocator, snap);
+    defer testing.allocator.free(rendered);
+    try testing.expect(std.mem.indexOf(u8, rendered, "kind::tax_type,label::Taxable") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered, "kind::tax_type,label::Roth (Post-Tax)") != null);
+    try testing.expect(std.mem.indexOf(u8, rendered, "kind::account,name::Emil Roth") != null);
+}
+
+test "renderSnapshot: front-matter emitted exactly once" {
+    // All four tail sections use emit_directives=false; only the meta
+    // call produces the #!srfv1 + #!created lines. Make sure we don't
+    // regress into duplicate headers.
+    const totals = [_]TotalRow{
+        .{ .kind = "total", .scope = "net_worth", .value = 1000 },
+    };
+    const snap: Snapshot = .{
+        .meta = .{
+            .kind = "meta",
+            .snapshot_version = 1,
+            .as_of_date = Date.fromYmd(2026, 4, 20),
+            .captured_at = 1,
+            .zfin_version = "x",
+            .stale_count = 0,
+        },
+        .totals = @constCast(&totals),
+        .tax_types = &.{},
+        .accounts = &.{},
+        .lots = &.{},
+    };
+    const rendered = try renderSnapshot(testing.allocator, snap);
+    defer testing.allocator.free(rendered);
+    try testing.expectEqual(@as(usize, 1), std.mem.count(u8, rendered, "#!srfv1"));
+    try testing.expectEqual(@as(usize, 1), std.mem.count(u8, rendered, "#!created="));
+}
diff --git a/src/main.zig b/src/main.zig
index 6a13ee3..487dfb1 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -19,6 +19,7 @@ const usage =
     \\  portfolio            Load and analyze the portfolio
     \\  analysis             Show portfolio analysis
     \\  contributions        Show money added since last commit (git-based diff)
+    \\  snapshot [opts]      Write a daily portfolio snapshot to history/
     \\  enrich <FILE|SYMBOL> Bootstrap metadata.srf from Alpha Vantage (25 req/day limit)
     \\  lookup <CUSIP>       Look up CUSIP to ticker via OpenFIGI
     \\  audit [opts]         Reconcile portfolio against brokerage export
@@ -229,6 +230,7 @@ pub fn main() !u8 {
         !std.mem.eql(u8, command, "analysis") and
         !std.mem.eql(u8, command, "contributions") and
         !std.mem.eql(u8, command, "portfolio") and
+        !std.mem.eql(u8, command, "snapshot") and
         !std.mem.eql(u8, command, "version");
     if (symbol_cmd and cmd_args.len >= 1) {
         for (cmd_args[0]) |*c| c.* = std.ascii.toUpper(c.*);
@@ -347,6 +349,13 @@ pub fn main() !u8 {
         const pf = resolveUserPath(allocator, config, globals.portfolio_path, zfin.Config.default_portfolio_filename);
         defer if (pf.resolved) |r| r.deinit(allocator);
         try commands.contributions.run(allocator, &svc, pf.path, color, out);
+    } else if (std.mem.eql(u8, command, "snapshot")) {
+        const pf = resolveUserPath(allocator, config, globals.portfolio_path, zfin.Config.default_portfolio_filename);
+        defer if (pf.resolved) |r| r.deinit(allocator);
+        commands.snapshot.run(allocator, &svc, pf.path, cmd_args, color, out) catch |err| switch (err) {
+            error.UnexpectedArg, error.PortfolioEmpty, error.WriteFailed => return 1,
+            else => return err,
+        };
     } else {
         try cli.stderrPrint("Unknown command. Run 'zfin help' for usage.\n");
         return 1;
@@ -418,6 +427,7 @@ const commands = struct {
     const audit = @import("commands/audit.zig");
     const enrich = @import("commands/enrich.zig");
     const contributions = @import("commands/contributions.zig");
+    const snapshot = @import("commands/snapshot.zig");
     const version = @import("commands/version.zig");
 };