diff --git a/src/clap/streaming.rs b/src/clap/streaming.rs
index d7f59518e819..c0b2e836a845 100644
--- a/src/clap/streaming.rs
+++ b/src/clap/streaming.rs
@@ -242,16 +242,17 @@ where
             } else {
                 false
             };
-            if param.takes_value == clap::Values::None
-                || param.takes_value == clap::Values::OneOptional
-            {
-                if next_is_eql && param.takes_value == clap::Values::None {
+            if param.takes_value == clap::Values::None {
+                if next_is_eql {
                     return Err(self.err(arg, Some(short), None, ArgError::DoesntTakeValue));
                 }
                 return Ok(Some(Arg { param, value: None }));
             }
 
             if arg.len() <= next_index {
+                if param.takes_value == clap::Values::OneOptional {
+                    return Ok(Some(Arg { param, value: None }));
+                }
                 let value = match self.iter.next() {
                     Some(v) => v,
                     None => {
@@ -554,6 +555,90 @@ mod tests {
         );
     }
 
+    #[test]
+    fn short_one_optional() {
+        let params: [clap::Param<u8>; 3] = [
+            clap::Param {
+                id: 0,
+                names: clap::Names {
+                    short: Some(b'a'),
+                    ..Default::default()
+                },
+                ..Default::default()
+            },
+            clap::Param {
+                id: 1,
+                names: clap::Names {
+                    short: Some(b'c'),
+                    ..Default::default()
+                },
+                takes_value: clap::Values::OneOptional,
+                ..Default::default()
+            },
+            clap::Param {
+                id: 2,
+                takes_value: clap::Values::One,
+                ..Default::default()
+            },
+        ];
+        let a = &params[0];
+        let c = &params[1];
+        let pos = &params[2];
+
+        test_no_err(
+            &params,
+            &[
+                b"-c", b"-c=v", b"-cv", b"-ac", b"-ac=v", b"-acv", b"-c", b"p",
+            ],
+            &[
+                Arg {
+                    param: c,
+                    value: None,
+                },
+                Arg {
+                    param: c,
+                    value: Some(b"v"),
+                },
+                Arg {
+                    param: c,
+                    value: Some(b"v"),
+                },
+                Arg {
+                    param: a,
+                    value: None,
+                },
+                Arg {
+                    param: c,
+                    value: None,
+                },
+                Arg {
+                    param: a,
+                    value: None,
+                },
+                Arg {
+                    param: c,
+                    value: Some(b"v"),
+                },
+                Arg {
+                    param: a,
+                    value: None,
+                },
+                Arg {
+                    param: c,
+                    value: Some(b"v"),
+                },
+                Arg {
+                    param: c,
+                    value: None,
+                },
+                Arg {
+                    param: pos,
+                    value: Some(b"p"),
+                },
+            ],
+        );
+    }
+
     #[test]
     fn long_params() {
         let params: [clap::Param<u8>; 4] = [
diff --git a/src/install/PackageManager/CommandLineArguments.rs b/src/install/PackageManager/CommandLineArguments.rs
index be895af25bf1..1d5711427668 100644
--- a/src/install/PackageManager/CommandLineArguments.rs
+++ b/src/install/PackageManager/CommandLineArguments.rs
@@ -51,7 +51,7 @@ const BACKEND_PARAM: ParamType = clap::param!(
 );
 
 const SHARED_HEAD_PARAMS: &[ParamType] = &[
-    clap::param!("-c, --config <STR>?                   Specify path to config file (bunfig.toml)"),
+    clap::param!("-c, --config <STR>                    Specify path to config file (bunfig.toml)"),
     clap::param!("-y, --yarn                            Write a yarn.lock file (yarn v1)"),
 ];
 
diff --git a/src/runtime/cli/Arguments.rs b/src/runtime/cli/Arguments.rs
index ab0fc39e9b37..e613e6fc2d4e 100644
--- a/src/runtime/cli/Arguments.rs
+++ b/src/runtime/cli/Arguments.rs
@@ -100,7 +100,7 @@ const BASE_PARAMS_: &[ParamType] = concat_params!(
             "--cwd <STR>                       Absolute path to resolve files & entry points from. This just changes the process' cwd."
         ),
         parse_param!(
-            "-c, --config <PATH>?              Specify path to Bun config file. Default <d>$cwd<r>/bunfig.toml"
+            "-c, --config <PATH>               Specify path to Bun config file. Default <d>$cwd<r>/bunfig.toml"
         ),
         parse_param!("-h, --help                        Display this menu and exit"),
     ],
diff --git a/test/cli/install/bun-run.test.ts b/test/cli/install/bun-run.test.ts
index 2dac7325e97c..3db076339da5 100644
--- a/test/cli/install/bun-run.test.ts
+++ b/test/cli/install/bun-run.test.ts
@@ -221,11 +221,10 @@ describe.concurrent("bun run", () => {
                   2,
                 ),
                 "index.js": "console.log('hi')",
-                ...(withLogLevel ? { "bunfig.toml": `logLevel = "debug"` } : {}),
+                "bunfig.toml": withLogLevel ? `logLevel = "debug"` : ``,
               });
 
               await using proc = Bun.spawn({
-                // TODO: figure out why -c is necessary here.
                 cmd: [
                   bunExe(),
                   ...(withRun ? ["run"] : []),
diff --git a/test/config/bunfig/config-flag.test.ts b/test/config/bunfig/config-flag.test.ts
new file mode 100644
index 000000000000..d52ed3807370
--- /dev/null
+++ b/test/config/bunfig/config-flag.test.ts
@@ -0,0 +1,99 @@
+import { describe, expect, test } from "bun:test";
+import { bunEnv, bunExe, tempDir } from "harness";
+import { join } from "path";
+
+// `-c` / `--config` must bind its path argument in every spelling.
+// https://github.com/oven-sh/bun/issues/6300, https://github.com/oven-sh/bun/issues/21431
+
+async function run(cwd: string, argv: string[]) {
+  await using proc = Bun.spawn({
+    cmd: [bunExe(), ...argv],
+    env: bunEnv,
+    cwd,
+    stdout: "pipe",
+    stderr: "pipe",
+  });
+  const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+  return { stdout, stderr, exitCode };
+}
+
+const spellings = [
+  ["--config=cfg.toml"],
+  ["--config", "cfg.toml"],
+  ["-c=cfg.toml"],
+  ["-c", "cfg.toml"],
+  ["-ccfg.toml"],
+];
+
+describe.concurrent("bun install --config path binding", () => {
+  test.each(spellings)("install %p loads the named config, never treats it as a package", async (...flag) => {
+    // Local 404 registry so nothing ever reaches the public network, even if
+    // parsing regresses and routes the path to `bun add`.
+    const hits: string[] = [];
+    await using server = Bun.serve({
+      port: 0,
+      fetch(req) {
+        hits.push(new URL(req.url).pathname);
+        return new Response("{}", { status: 404 });
+      },
+    });
+    const base = `http://localhost:${server.port}`;
+
+    using dir = tempDir("config-flag-install", {
+      "package.json": JSON.stringify({ name: "x", version: "1.0.0", dependencies: { "no-deps": "1.0.0" } }),
+      // Auto-loaded when `--config` is NOT effective (regression).
+      "bunfig.toml": `[install]\nregistry = "${base}/default/"\ncache = false\n`,
+      // Loaded when `--config cfg.toml` IS effective (the fix).
+      "cfg.toml": `[install]\nregistry = "${base}/fromcfg/"\ncache = false\n`,
+    });
+
+    const { stdout, stderr, exitCode } = await run(String(dir), ["install", ...flag]);
+    const out = stdout + stderr;
+
+    // The path must never be routed to `bun add` as a package name.
+    expect(out).not.toContain("bun add");
+    expect(hits).not.toContain("/default/cfg.toml");
+    // The named config must have been loaded, not the auto-loaded bunfig.toml.
+    expect(hits).toContain("/fromcfg/no-deps");
+    expect(hits).not.toContain("/default/no-deps");
+    expect(exitCode).not.toBe(0);
+  });
+});
+
+describe.concurrent("bare -c / --config requires a value", () => {
+  test.each([[["install", "--config"]], [["install", "-c"]], [["--config"]], [["-c"]]])(
+    "bun %p errors instead of silently defaulting",
+    async argv => {
+      using dir = tempDir("config-flag-bare", {
+        "package.json": JSON.stringify({ name: "x", version: "1.0.0" }),
+      });
+      const { stderr, exitCode } = await run(String(dir), argv);
+      expect(stderr).toContain(argv.at(-1)!);
+      expect(stderr.toLowerCase()).toContain("requires a value");
+      expect(exitCode).not.toBe(0);
+    },
+  );
+});
+
+// The subcommand keyword is picked before flag values are parsed, so only the
+// single-token spellings can precede `run`; the space forms go after it.
+const scriptArgv = [
+  ...spellings,
+  ...spellings.map(flag => ["run", ...flag]),
+  ...spellings.filter(flag => flag.length === 1).map(flag => [...flag, "run"]),
+];
+
+describe.concurrent("bun [run] --config path binding", () => {
+  test.each(scriptArgv.map(argv => [argv]))("bun %p app.ts loads the named config and runs app.ts", async argv => {
+    using dir = tempDir("config-flag-run", {
+      "app.ts": `console.log(JSON.stringify({ fromCfg: process.env.FROM_CFG ?? "no", argv: process.argv.slice(2) }));`,
+      "cfg.toml": `[define]\n"process.env.FROM_CFG" = '"yes"'\n`,
+    });
+
+    const { stdout, stderr, exitCode } = await run(String(dir), [...argv, join(String(dir), "app.ts"), "pass-through"]);
+    // The config path must not be treated as the entry point.
+    expect(stderr).not.toContain("cfg.toml");
+    expect(JSON.parse(stdout.trim())).toEqual({ fromCfg: "yes", argv: ["pass-through"] });
+    expect(exitCode).toBe(0);
+  });
+});