From f2076e38b4f636bbd993dc391b2b09d4638e0c53 Mon Sep 17 00:00:00 2001
From: robobun <117481402+robobun@users.noreply.github.com>
Date: Mon, 11 May 2026 04:45:13 +0000
Subject: [PATCH] bundler: fix ESMConditions capacity when user conditions are
 provided

The expression `if (allow_addons) 1 else 0 + conditions.len` parses as
`if (allow_addons) 1 else (0 + conditions.len)`, so when allow_addons is
true the user-provided conditions were not counted towards the reserved
capacity. With enough custom conditions this tripped the
addOneAssumeCapacity assertion in debug builds and wrote past the
allocation in release builds.
---
 src/bundler/options.zig            |  7 ++++---
 test/bundler/bun-build-api.test.ts | 18 ++++++++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/src/bundler/options.zig b/src/bundler/options.zig
index daea5ced59a3..19075362b889 100644
--- a/src/bundler/options.zig
+++ b/src/bundler/options.zig
@@ -1087,9 +1087,10 @@ pub const ESMConditions = struct {
         var require_condition_map = ConditionsMap.init(allocator);
         var style_condition_map = ConditionsMap.init(allocator);
 
-        try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
-        try import_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
-        try require_condition_map.ensureTotalCapacity(defaults.len + 2 + if (allow_addons) 1 else 0 + conditions.len);
+        const extra: usize = if (allow_addons) 1 else 0;
+        try default_condition_amp.ensureTotalCapacity(defaults.len + 2 + extra + conditions.len);
+        try import_condition_map.ensureTotalCapacity(defaults.len + 2 + extra + conditions.len);
+        try require_condition_map.ensureTotalCapacity(defaults.len + 2 + extra + conditions.len);
         try style_condition_map.ensureTotalCapacity(defaults.len + 2 + conditions.len);
 
         import_condition_map.putAssumeCapacity("import", {});
diff --git a/test/bundler/bun-build-api.test.ts b/test/bundler/bun-build-api.test.ts
index c1112743f02c..af6526ad1382 100644
--- a/test/bundler/bun-build-api.test.ts
+++ b/test/bundler/bun-build-api.test.ts
@@ -811,6 +811,24 @@ identity(mod23);
   expect(text).toContain(" globalThis.");
 });
 
+test("many custom conditions does not crash", async () => {
+  const dir = tempDirWithFiles("bun-build-many-conditions", {
+    "entry.js": `console.log(1);`,
+  });
+
+  for (const target of ["bun", "browser", "node"] as const) {
+    for (const n of [1, 4, 6, 8, 12, 20]) {
+      const conditions = Array.from({ length: n }, (_, i) => `cond${i}`);
+      const build = await Bun.build({
+        entrypoints: [join(dir, "entry.js")],
+        target,
+        conditions,
+      });
+      expect(build.success).toBe(true);
+    }
+  }
+});
+
 describe.concurrent("sourcemap boolean values", () => {
   test("sourcemap: true should work (boolean)", async () => {
     const dir = tempDirWithFiles("sourcemap-true-boolean", {