diff --git a/src/cache/store.zig b/src/cache/store.zig
index 232df2f..3d82557 100644
--- a/src/cache/store.zig
+++ b/src/cache/store.zig
@@ -1,6 +1,7 @@
 const std = @import("std");
 const log = std.log.scoped(.cache);
 const srf = @import("srf");
+const atomic = @import("../atomic.zig");
 const Date = @import("../models/date.zig").Date;
 const Candle = @import("../models/candle.zig").Candle;
 const Dividend = @import("../models/dividend.zig").Dividend;
@@ -428,26 +429,58 @@ pub const Store = struct {
 
     /// Write raw bytes to a cache file. Used by server sync to write
     /// pre-serialized SRF data directly to the cache.
+    ///
+    /// Atomic: writes to `<path>.tmp`, fsyncs, and renames. A concurrent
+    /// reader sees either the old complete file or the new complete file —
+    /// never a truncated-mid-write state. This matters because:
+    ///
+    ///   - `parallelServerSync` (service.zig) writes many cache files in
+    ///     parallel; other code paths in the same process (TUI redraws,
+    ///     progress callbacks that happen to peek, future concurrent
+    ///     readers) must never observe a half-written file.
+    ///   - The prior implementation used `createFile(truncate) + writeAll`,
+    ///     which has a window between truncation and write-completion
+    ///     where a reader gets 0..data.len bytes. The symptom was SRF
+    ///     `custom parse of value 2026-04 failed : InvalidDateFormat`
+    ///     — a date field truncated exactly 7 chars into its 10-char
+    ///     value.
     pub fn writeRaw(self: *Store, symbol: []const u8, data_type: DataType, data: []const u8) !void {
         try self.ensureSymbolDir(symbol);
         const path = try self.symbolPath(symbol, data_type.fileName());
         defer self.allocator.free(path);
 
-        const file = try std.fs.cwd().createFile(path, .{});
-        defer file.close();
-        try file.writeAll(data);
+        try atomic.writeFileAtomic(self.allocator, path, data);
     }
 
+    /// Append raw bytes to an existing cache file.
+    ///
+    /// Implemented as read-existing + concat + atomic write rather than
+    /// a direct open-for-append, for the same reason as `writeRaw`:
+    /// a true append (`seekFromEnd + writeAll`) leaves a reader that hits
+    /// the file mid-append with a valid head + partial tail, which for
+    /// SRF data means a truncated trailing record. With atomic rewrite,
+    /// the rename primitive guarantees readers see either the pre-append
+    /// state or the post-append state, never an in-between.
+    ///
+    /// Returns `error.FileNotFound` if the target doesn't exist yet —
+    /// callers are expected to fall back to a full rewrite path in that
+    /// case (see `appendCandles`).
     fn appendRaw(self: *Store, symbol: []const u8, data_type: DataType, data: []const u8) !void {
         const path = try self.symbolPath(symbol, data_type.fileName());
         defer self.allocator.free(path);
 
-        const file = std.fs.cwd().openFile(path, .{ .mode = .write_only }) catch {
-            return error.FileNotFound;
+        const existing = std.fs.cwd().readFileAlloc(self.allocator, path, 50 * 1024 * 1024) catch |err| switch (err) {
+            error.FileNotFound => return error.FileNotFound,
+            else => return err,
         };
-        defer file.close();
-        try file.seekFromEnd(0);
-        try file.writeAll(data);
+        defer self.allocator.free(existing);
+
+        const combined = try self.allocator.alloc(u8, existing.len + data.len);
+        defer self.allocator.free(combined);
+        @memcpy(combined[0..existing.len], existing);
+        @memcpy(combined[existing.len..], data);
+
+        try atomic.writeFileAtomic(self.allocator, path, combined);
     }
 
     fn symbolPath(self: *Store, symbol: []const u8, file_name: []const u8) ![]const u8 {
@@ -1060,3 +1093,224 @@ test "CandleMeta default provider is tiingo" {
     };
     try std.testing.expectEqual(Store.CandleProvider.tiingo, meta.provider);
 }
+
+// ── writeRaw / appendRaw atomicity ───────────────────────────
+//
+// A concurrent reader hitting a cache file mid-write must never see a
+// truncated or partial-field state — the symptom was srf `custom parse
+// of value 2026-04 failed : InvalidDateFormat`, a date field chopped
+// exactly 7 chars into a 10-char value.
+//
+// These tests exercise the atomicity primitive underneath writeRaw/
+// appendRaw: while one thread hammers writes of two alternating,
+// differently-sized SRF blobs, reader threads hammer reads and assert
+// that every read they succeed at parses cleanly as one of those two
+// blobs — no partial content, no partial dates. A pre-atomic-fix
+// version of writeRaw would fail this test within a handful of
+// iterations.
+
+const testing = std.testing;
+
+test "writeRaw atomicity: concurrent readers never observe a truncated file" {
+    // Two SRF blobs with dates at different byte offsets. A non-atomic
+    // writer that truncates + writes would leak partial bytes of
+    // whichever blob is mid-write; that would show up as either a
+    // partial date or a split field — both caught by strict equality
+    // against these two complete blobs.
+    const blob_a =
+        \\#!srfv1
+        \\#!expires=1780000000
+        \\#!created=1777000000
+        \\symbol::AAPL,date::2026-04-30,actual:num:2.78
+        \\symbol::MSFT,date::2026-01-29,actual:num:4.27
+        \\
+    ;
+    const blob_b =
+        \\#!srfv1
+        \\#!expires=1780500000
+        \\#!created=1777500000
+        \\symbol::AMZN,date::2026-07-29,estimate:num:1.83
+        \\
+    ;
+    try std.testing.expect(blob_a.len != blob_b.len); // ensure size changes
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    const dir_path = try tmp.dir.realpathAlloc(testing.allocator, ".");
+    defer testing.allocator.free(dir_path);
+
+    // Use a thread-safe allocator because multiple worker threads
+    // allocate through Store/writeFileAtomic concurrently.
+    var thread_safe: std.heap.ThreadSafeAllocator = .{ .child_allocator = testing.allocator };
+    const alloc = thread_safe.allocator();
+
+    var store = Store.init(alloc, dir_path);
+
+    // Make sure the symbol subdir exists once up-front (writeRaw will
+    // also call ensureSymbolDir, but doing it here keeps the writer
+    // hot loop lean).
+    try store.ensureSymbolDir("SYM");
+
+    const Ctx = struct {
+        store: *Store,
+        blob_a: []const u8,
+        blob_b: []const u8,
+        stop: std.atomic.Value(bool),
+        bad_reads: std.atomic.Value(u32),
+        completed_reads: std.atomic.Value(u32),
+
+        fn writer(self: *@This()) void {
+            var i: usize = 0;
+            while (!self.stop.load(.acquire)) : (i += 1) {
+                const bytes = if (i & 1 == 0) self.blob_a else self.blob_b;
+                // We don't care about errors here — worst case the
+                // writer just skips this iteration.
+                self.store.writeRaw("SYM", .candles_daily, bytes) catch {};
+            }
+        }
+
+        fn reader(self: *@This()) void {
+            const path = self.store.symbolPath("SYM", DataType.candles_daily.fileName()) catch return;
+            defer self.store.allocator.free(path);
+
+            while (!self.stop.load(.acquire)) {
+                const bytes = std.fs.cwd().readFileAlloc(self.store.allocator, path, 1 * 1024 * 1024) catch |err| switch (err) {
+                    error.FileNotFound => continue, // pre-first-write race
+                    else => continue,
+                };
+                defer self.store.allocator.free(bytes);
+
+                // Every successful read must match exactly one of the
+                // two source blobs. A non-atomic writer would eventually
+                // produce reads that match neither (truncated, zero
+                // bytes, or mid-blob switchover garbage).
+                const matches = std.mem.eql(u8, bytes, self.blob_a) or std.mem.eql(u8, bytes, self.blob_b);
+                if (!matches) _ = self.bad_reads.fetchAdd(1, .monotonic);
+                _ = self.completed_reads.fetchAdd(1, .monotonic);
+            }
+        }
+    };
+
+    var ctx = Ctx{
+        .store = &store,
+        .blob_a = blob_a,
+        .blob_b = blob_b,
+        .stop = std.atomic.Value(bool).init(false),
+        .bad_reads = std.atomic.Value(u32).init(0),
+        .completed_reads = std.atomic.Value(u32).init(0),
+    };
+
+    var writer_thread = try std.Thread.spawn(.{}, Ctx.writer, .{&ctx});
+    var reader_threads: [3]std.Thread = undefined;
+    for (&reader_threads) |*t| t.* = try std.Thread.spawn(.{}, Ctx.reader, .{&ctx});
+
+    // Run the stress for a fixed duration. 200ms is plenty for thousands
+    // of iterations on any reasonable machine — enough to reliably catch
+    // a non-atomic write window at the scheduler granularity.
+    std.Thread.sleep(200 * std.time.ns_per_ms);
+    ctx.stop.store(true, .release);
+
+    writer_thread.join();
+    for (&reader_threads) |*t| t.join();
+
+    // The critical assertion: zero reads landed in a partial state.
+    const bad = ctx.bad_reads.load(.monotonic);
+    const total = ctx.completed_reads.load(.monotonic);
+
+    // Sanity: we actually did work (otherwise the test is a no-op).
+    try testing.expect(total > 0);
+    try testing.expectEqual(@as(u32, 0), bad);
+}
+
+test "appendRaw atomicity: concurrent readers see either pre- or post-append, never mid" {
+    // Seed an initial file with a complete SRF doc, then have one thread
+    // append more records repeatedly while readers race to read it. Every
+    // successful read must parse cleanly and have a valid termination
+    // — no trailing partial record from an in-flight append.
+    const seed =
+        \\#!srfv1
+        \\#!created=1777000000
+        \\symbol::AAPL,date::2024-04-30,actual:num:1.53
+        \\
+    ;
+    const chunk =
+        \\symbol::AAPL,date::2024-07-30,actual:num:1.65
+        \\
+    ;
+
+    var tmp = std.testing.tmpDir(.{});
+    defer tmp.cleanup();
+
+    const dir_path = try tmp.dir.realpathAlloc(testing.allocator, ".");
+    defer testing.allocator.free(dir_path);
+
+    var thread_safe: std.heap.ThreadSafeAllocator = .{ .child_allocator = testing.allocator };
+    const alloc = thread_safe.allocator();
+
+    var store = Store.init(alloc, dir_path);
+    try store.ensureSymbolDir("SYM");
+
+    // Write seed atomically up front.
+    try store.writeRaw("SYM", .candles_daily, seed);
+
+    const Ctx = struct {
+        store: *Store,
+        chunk: []const u8,
+        stop: std.atomic.Value(bool),
+        bad_reads: std.atomic.Value(u32),
+        completed_reads: std.atomic.Value(u32),
+
+        fn appender(self: *@This()) void {
+            while (!self.stop.load(.acquire)) {
+                self.store.appendRaw("SYM", .candles_daily, self.chunk) catch {};
+            }
+        }
+
+        fn reader(self: *@This()) void {
+            const path = self.store.symbolPath("SYM", DataType.candles_daily.fileName()) catch return;
+            defer self.store.allocator.free(path);
+
+            while (!self.stop.load(.acquire)) {
+                const bytes = std.fs.cwd().readFileAlloc(self.store.allocator, path, 4 * 1024 * 1024) catch continue;
+                defer self.store.allocator.free(bytes);
+
+                // Invariants for a valid appended file:
+                //  1. Starts with `#!srfv1\n` (the header is never torn).
+                //  2. Ends with `\n` (no partial trailing record).
+                //  3. Total length is `seed.len + k * chunk.len` for some
+                //     integer k ≥ 0 — which, with atomic appends, is the
+                //     only observable shape.
+                const ok_header = std.mem.startsWith(u8, bytes, "#!srfv1\n");
+                const ok_tail = bytes.len > 0 and bytes[bytes.len - 1] == '\n';
+                if (!ok_header or !ok_tail) {
+                    _ = self.bad_reads.fetchAdd(1, .monotonic);
+                }
+                _ = self.completed_reads.fetchAdd(1, .monotonic);
+            }
+        }
+    };
+
+    var ctx = Ctx{
+        .store = &store,
+        .chunk = chunk,
+        .stop = std.atomic.Value(bool).init(false),
+        .bad_reads = std.atomic.Value(u32).init(0),
+        .completed_reads = std.atomic.Value(u32).init(0),
+    };
+
+    var appender_thread = try std.Thread.spawn(.{}, Ctx.appender, .{&ctx});
+    var reader_threads: [3]std.Thread = undefined;
+    for (&reader_threads) |*t| t.* = try std.Thread.spawn(.{}, Ctx.reader, .{&ctx});
+
+    std.Thread.sleep(200 * std.time.ns_per_ms);
+    ctx.stop.store(true, .release);
+
+    appender_thread.join();
+    for (&reader_threads) |*t| t.join();
+
+    const bad = ctx.bad_reads.load(.monotonic);
+    const total = ctx.completed_reads.load(.monotonic);
+    try testing.expect(total > 0);
+    try testing.expectEqual(@as(u32, 0), bad);
+}
diff --git a/src/service.zig b/src/service.zig
index a0e238f..36c585e 100644
--- a/src/service.zig
+++ b/src/service.zig
@@ -9,6 +9,7 @@
 
 const std = @import("std");
 const log = std.log.scoped(.service);
+
 const Date = @import("models/date.zig").Date;
 const Candle = @import("models/candle.zig").Candle;
 const Dividend = @import("models/dividend.zig").Dividend;
@@ -33,6 +34,7 @@ const Tiingo = @import("providers/tiingo.zig").Tiingo;
 const fmt = @import("format.zig");
 const performance = @import("analytics/performance.zig");
 const http = @import("net/http.zig");
+const atomic = @import("atomic.zig");
 
 pub const DataError = error{
     NoApiKey,
@@ -1296,6 +1298,11 @@ pub const DataService = struct {
     }
 
     /// Append a CUSIP->ticker mapping to the cache file.
+    ///
+    /// Implemented as read-append-atomic-write (rather than a direct
+    /// open-for-append) so a concurrent reader never sees a file with a
+    /// valid header plus partial trailing record. See `cache/store.zig
+    /// appendRaw` for the same pattern and rationale.
     pub fn cacheCusipTicker(self: *DataService, cusip: []const u8, ticker: []const u8) void {
         const path = std.fs.path.join(self.allocator(), &.{ self.config.cache_dir, "cusip_tickers.srf" }) catch return;
         defer self.allocator().free(path);
@@ -1305,20 +1312,31 @@ pub const DataService = struct {
             std.fs.cwd().makePath(dir) catch {};
         }
 
-        // Open existing (append) or create new (with header)
-        var emit_directives = false;
-        const file = std.fs.cwd().openFile(path, .{ .mode = .write_only }) catch blk: {
-            emit_directives = true;
-            break :blk std.fs.cwd().createFile(path, .{}) catch return;
+        // Read existing cache if present.
+        const existing = std.fs.cwd().readFileAlloc(self.allocator(), path, 4 * 1024 * 1024) catch |err| switch (err) {
+            error.FileNotFound => @as([]u8, &.{}),
+            else => return,
         };
-        defer file.close();
-        if (!emit_directives) file.seekFromEnd(0) catch {};
+        const owns_existing = existing.len > 0;
+        defer if (owns_existing) self.allocator().free(existing);
 
+        // Serialize the new entry (with `#!srfv1` directives only if the
+        // cache file doesn't exist yet).
+        const emit_directives = !owns_existing;
         const entry = [_]CusipEntry{.{ .cusip = cusip, .ticker = ticker }};
-        var buf: [256]u8 = undefined;
-        var writer = file.writer(&buf);
-        writer.interface.print("{f}", .{srf.fmtFrom(CusipEntry, self.allocator(), &entry, .{ .emit_directives = emit_directives })}) catch return;
-        writer.interface.flush() catch {};
+        var aw: std.Io.Writer.Allocating = .init(self.allocator());
+        defer aw.deinit();
+        aw.writer.print("{f}", .{srf.fmtFrom(CusipEntry, self.allocator(), &entry, .{ .emit_directives = emit_directives })}) catch return;
+        const encoded = aw.writer.buffered();
+        if (encoded.len == 0) return;
+
+        // Concat existing + new, then atomic-write.
+        const combined = self.allocator().alloc(u8, existing.len + encoded.len) catch return;
+        defer self.allocator().free(combined);
+        @memcpy(combined[0..existing.len], existing);
+        @memcpy(combined[existing.len..], encoded);
+
+        atomic.writeFileAtomic(self.allocator(), path, combined) catch {};
     }
 
     // ── Utility ──────────────────────────────────────────────────