diff --git a/Cargo.lock b/Cargo.lock
index 451ee4215d..8a08f5f2e8 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2033,6 +2033,7 @@ dependencies = [
  "oxc_ast",
  "oxc_diagnostics",
  "oxc_formatter_core",
+ "oxc_formatter_tests",
  "oxc_jsdoc",
  "oxc_parser",
  "oxc_semantic",
@@ -2058,7 +2059,6 @@ dependencies = [
  "oxc_data_structures",
  "oxc_span",
  "rustc-hash",
- "serde_json",
  "unicode-width",
 ]
 
@@ -2072,6 +2072,7 @@ dependencies = [
  "oxc_allocator",
  "oxc_diagnostics",
  "oxc_formatter_core",
+ "oxc_formatter_tests",
  "oxc_span",
  "phf",
  "pico-args",
@@ -2087,6 +2088,7 @@ dependencies = [
  "oxc_allocator",
  "oxc_diagnostics",
  "oxc_formatter_core",
+ "oxc_formatter_tests",
  "oxc_span",
  "oxc_tasks_common",
  "pico-args",
@@ -2102,6 +2104,7 @@ dependencies = [
  "oxc_ast",
  "oxc_diagnostics",
  "oxc_formatter_core",
+ "oxc_formatter_tests",
  "oxc_parser",
  "oxc_span",
  "oxc_syntax",
@@ -2109,6 +2112,14 @@ dependencies = [
  "pico-args",
 ]
 
+[[package]]
+name = "oxc_formatter_tests"
+version = "0.62.0"
+dependencies = [
+ "oxc_formatter_core",
+ "serde_json",
+]
+
 [[package]]
 name = "oxc_formatter_yaml"
 version = "0.63.0"
@@ -2119,6 +2130,7 @@ dependencies = [
  "oxc_allocator",
  "oxc_diagnostics",
  "oxc_formatter_core",
+ "oxc_formatter_tests",
  "oxc_span",
  "oxc_tasks_common",
  "pico-args",
diff --git a/Cargo.toml b/Cargo.toml
index 9b04b9d30b..af1bf3073d 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -163,6 +163,7 @@ oxc_formatter_core = { path = "crates/oxc_formatter_core" } # Language-agnostic
 oxc_formatter_css = { path = "crates/oxc_formatter_css" } # CSS formatter
 oxc_formatter_graphql = { path = "crates/oxc_formatter_graphql" } # GraphQL formatter
 oxc_formatter_json = { path = "crates/oxc_formatter_json" } # JSON formatter
+oxc_formatter_tests = { path = "crates/oxc_formatter_tests" } # Formatter test infrastructure (fixture codegen + harness)
 oxc_formatter_yaml = { path = "crates/oxc_formatter_yaml" } # YAML formatter
 oxc_language_server = { path = "crates/oxc_language_server", default-features = false } # Language server
 oxc_lexer = { path = "crates/oxc_lexer" } # JS/TS lexer (incubating; x86_64 SIMD, stub elsewhere)
diff --git a/crates/oxc_formatter/Cargo.toml b/crates/oxc_formatter/Cargo.toml
index b3375aa179..535b02ae90 100644
--- a/crates/oxc_formatter/Cargo.toml
+++ b/crates/oxc_formatter/Cargo.toml
@@ -41,14 +41,14 @@ unicode-width = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 oxc_tasks_common = { workspace = true }
 pico-args = { workspace = true }
 serde_json = { workspace = true }
 serde = { workspace = true, features = ["derive"] }
 
 [build-dependencies]
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 
 [features]
 default = []
diff --git a/crates/oxc_formatter/build.rs b/crates/oxc_formatter/build.rs
index c0a3dc8904..94b76fcef1 100644
--- a/crates/oxc_formatter/build.rs
+++ b/crates/oxc_formatter/build.rs
@@ -3,7 +3,7 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use oxc_formatter_core::test_support::{GenerateConfig, generate_tests};
+use oxc_formatter_tests::{GenerateConfig, generate_tests};
 
 fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
diff --git a/crates/oxc_formatter/tests/fixtures/mod.rs b/crates/oxc_formatter/tests/fixtures/mod.rs
index 3c1b45c089..ba211d8e41 100644
--- a/crates/oxc_formatter/tests/fixtures/mod.rs
+++ b/crates/oxc_formatter/tests/fixtures/mod.rs
@@ -5,7 +5,7 @@ use oxc_formatter::{
     ArrowParentheses, BracketSameLine, BracketSpacing, JsFormatOptions, JsdocOptions,
     QuoteProperties, QuoteStyle, Semicolons, TrailingCommas,
 };
-use oxc_formatter_core::test_support::{
+use oxc_formatter_tests::{
     FixtureFormatter, OptionSet, apply_core_options, build_fixture_snapshot,
 };
 use oxc_span::SourceType;
diff --git a/crates/oxc_formatter_core/AGENTS.md b/crates/oxc_formatter_core/AGENTS.md
index 183473a06c..b9156881af 100644
--- a/crates/oxc_formatter_core/AGENTS.md
+++ b/crates/oxc_formatter_core/AGENTS.md
@@ -160,18 +160,11 @@ core owns the span-ordered cursor mechanics; what the items mean (comments) and
 
 Quote-style options, comment rules, and the like are likewise consumer-owned.
 
-## Cargo features
-
-`test_harness` exposes `test_support/` (fixture test generation) to downstream crates.
-Consumers still need their own `insta` dev-dep so the recorded `source:` header points to the consumer crate.
-
 ## Verification
 
-Feature configs to check:
-
 ```sh
 cargo c -p oxc_formatter_core
-cargo c -p oxc_formatter_core --features test_harness
 ```
 
 This crate has basic tests only of its own, it is exercised through the conformance/snapshot tests of its consumers.
+The fixture-test infrastructure those consumers use lives in `crates/oxc_formatter_tests`.
diff --git a/crates/oxc_formatter_core/Cargo.toml b/crates/oxc_formatter_core/Cargo.toml
index 1fe82db1bb..e331830fb7 100644
--- a/crates/oxc_formatter_core/Cargo.toml
+++ b/crates/oxc_formatter_core/Cargo.toml
@@ -25,14 +25,5 @@ cow-utils = { workspace = true }
 rustc-hash = { workspace = true }
 unicode-width = { workspace = true }
 
-serde_json = { workspace = true, optional = true }
-
-[features]
-# Enables `test_support` (used from build.rs and tests/) in downstream formatter
-# crates. Consumer crates still need a direct `insta` dev-dep — the snapshot macro
-# is invoked from each consumer's `tests/fixtures/mod.rs` so the recorded `source:`
-# header points to the consumer, not the shared harness.
-test_harness = ["dep:serde_json"]
-
 [lib]
 doctest = false
diff --git a/crates/oxc_formatter_core/src/lib.rs b/crates/oxc_formatter_core/src/lib.rs
index 71a28d1a40..f08fa4b8c0 100644
--- a/crates/oxc_formatter_core/src/lib.rs
+++ b/crates/oxc_formatter_core/src/lib.rs
@@ -35,9 +35,6 @@ pub mod spec;
 mod state;
 mod traits;
 
-#[cfg(feature = "test_harness")]
-pub mod test_support;
-
 pub use arguments::{Argument, Arguments};
 pub use buffer::{
     AccumulatorBuffer, Buffer, BufferExtensions, HeapVecBuffer, Inspect, PreambleBuffer, Recorded,
diff --git a/crates/oxc_formatter_css/Cargo.toml b/crates/oxc_formatter_css/Cargo.toml
index bf73abead4..6a868e67eb 100644
--- a/crates/oxc_formatter_css/Cargo.toml
+++ b/crates/oxc_formatter_css/Cargo.toml
@@ -27,11 +27,11 @@ oxc-css-parser = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 pico-args = { workspace = true }
 
 [build-dependencies]
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 
 [lib]
 doctest = false
diff --git a/crates/oxc_formatter_css/build.rs b/crates/oxc_formatter_css/build.rs
index b8c14f5a5e..cfc0b37329 100644
--- a/crates/oxc_formatter_css/build.rs
+++ b/crates/oxc_formatter_css/build.rs
@@ -3,7 +3,7 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use oxc_formatter_core::test_support::{GenerateConfig, generate_tests};
+use oxc_formatter_tests::{GenerateConfig, generate_tests};
 
 fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
diff --git a/crates/oxc_formatter_css/tests/fixtures/mod.rs b/crates/oxc_formatter_css/tests/fixtures/mod.rs
index b930f97626..e177c97645 100644
--- a/crates/oxc_formatter_css/tests/fixtures/mod.rs
+++ b/crates/oxc_formatter_css/tests/fixtures/mod.rs
@@ -6,10 +6,10 @@
 use std::path::Path;
 
 use oxc_allocator::{Allocator, ArenaVec};
-use oxc_formatter_core::test_support::{
+use oxc_formatter_css::{CssFormatOptions, CssVariant, format};
+use oxc_formatter_tests::{
     FixtureFormatter, OptionSet, apply_core_options, build_fixture_snapshot,
 };
-use oxc_formatter_css::{CssFormatOptions, CssVariant, format};
 
 struct CssHarness;
 
diff --git a/crates/oxc_formatter_graphql/Cargo.toml b/crates/oxc_formatter_graphql/Cargo.toml
index c52073f873..9f0c034deb 100644
--- a/crates/oxc_formatter_graphql/Cargo.toml
+++ b/crates/oxc_formatter_graphql/Cargo.toml
@@ -26,12 +26,12 @@ oxc_span = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 oxc_tasks_common = { workspace = true }
 pico-args = { workspace = true }
 
 [build-dependencies]
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 
 [lib]
 doctest = false
diff --git a/crates/oxc_formatter_graphql/build.rs b/crates/oxc_formatter_graphql/build.rs
index 098c0ba764..eee2baa8bf 100644
--- a/crates/oxc_formatter_graphql/build.rs
+++ b/crates/oxc_formatter_graphql/build.rs
@@ -3,7 +3,7 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use oxc_formatter_core::test_support::{GenerateConfig, generate_tests};
+use oxc_formatter_tests::{GenerateConfig, generate_tests};
 
 fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
diff --git a/crates/oxc_formatter_graphql/tests/fixtures/mod.rs b/crates/oxc_formatter_graphql/tests/fixtures/mod.rs
index e71ef3eb1d..85aa7d4675 100644
--- a/crates/oxc_formatter_graphql/tests/fixtures/mod.rs
+++ b/crates/oxc_formatter_graphql/tests/fixtures/mod.rs
@@ -1,10 +1,10 @@
 use std::path::Path;
 
 use oxc_allocator::Allocator;
-use oxc_formatter_core::test_support::{
+use oxc_formatter_graphql::{GraphqlFormatOptions, format};
+use oxc_formatter_tests::{
     FixtureFormatter, OptionSet, apply_core_options, build_fixture_snapshot,
 };
-use oxc_formatter_graphql::{GraphqlFormatOptions, format};
 
 struct GraphqlHarness;
 
diff --git a/crates/oxc_formatter_json/Cargo.toml b/crates/oxc_formatter_json/Cargo.toml
index 6d6f882da2..590fcf36b8 100644
--- a/crates/oxc_formatter_json/Cargo.toml
+++ b/crates/oxc_formatter_json/Cargo.toml
@@ -28,12 +28,12 @@ oxc_syntax = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 oxc_tasks_common = { workspace = true }
 pico-args = { workspace = true }
 
 [build-dependencies]
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 
 [lib]
 doctest = false
diff --git a/crates/oxc_formatter_json/build.rs b/crates/oxc_formatter_json/build.rs
index 5e8e9a0cc2..cd9cb7cc02 100644
--- a/crates/oxc_formatter_json/build.rs
+++ b/crates/oxc_formatter_json/build.rs
@@ -3,7 +3,7 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use oxc_formatter_core::test_support::{GenerateConfig, generate_tests};
+use oxc_formatter_tests::{GenerateConfig, generate_tests};
 
 fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
diff --git a/crates/oxc_formatter_json/tests/fixtures/mod.rs b/crates/oxc_formatter_json/tests/fixtures/mod.rs
index 0714928872..d2f697f125 100644
--- a/crates/oxc_formatter_json/tests/fixtures/mod.rs
+++ b/crates/oxc_formatter_json/tests/fixtures/mod.rs
@@ -1,12 +1,12 @@
 use std::path::Path;
 
 use oxc_allocator::Allocator;
-use oxc_formatter_core::test_support::{
-    FixtureFormatter, OptionSet, apply_core_options, build_fixture_snapshot,
-};
 use oxc_formatter_json::{
     BracketSpacing, Expand, JsonFormatOptions, JsonVariant, QuoteProps, TrailingCommas, format,
 };
+use oxc_formatter_tests::{
+    FixtureFormatter, OptionSet, apply_core_options, build_fixture_snapshot,
+};
 
 struct JsonHarness;
 
diff --git a/crates/oxc_formatter_tests/AGENTS.md b/crates/oxc_formatter_tests/AGENTS.md
new file mode 100644
index 0000000000..b8b7ee90fe
--- /dev/null
+++ b/crates/oxc_formatter_tests/AGENTS.md
@@ -0,0 +1,24 @@
+# Coding agent guides for `crates/oxc_formatter_tests`
+
+Test infrastructure shared by the formatter crates (e.g. `oxc_formatter`, `oxc_formatter_json`, `oxc_formatter_css`, etc).
+
+- `codegen`: build-script helper — consumers call `generate_tests` from `build.rs` via `[build-dependencies]` to emit one `#[test]` per fixture file
+- `harness`: fixture runtime — consumers implement `FixtureFormatter` in `tests/fixtures/mod.rs` via `[dev-dependencies]`
+
+## Dependency rule
+
+This crate depends only on `oxc_formatter_core` — NEVER on language crates.
+Consumers use it from both `[build-dependencies]` and `[dev-dependencies]`; a dependency on a language crate would make that a build-dep cycle, which cargo rejects.
+
+## insta stays consumer-side
+
+`build_fixture_snapshot` returns the snapshot body without calling `insta::assert_snapshot!`.
+Each consumer invokes the macro from its own `tests/fixtures/mod.rs` so the recorded `source:` header points to the consumer crate (required by `INSTA_REQUIRE_FULL_MATCH=1` in CI). Consumers therefore need their own `insta` dev-dep.
+
+## Verification
+
+```sh
+cargo c -p oxc_formatter_tests
+```
+
+Exercised through the fixture tests of its consumers (`cargo test -p <formatter crate>`).
diff --git a/crates/oxc_formatter_tests/CLAUDE.md b/crates/oxc_formatter_tests/CLAUDE.md
new file mode 100644
index 0000000000..43c994c2d3
--- /dev/null
+++ b/crates/oxc_formatter_tests/CLAUDE.md
@@ -0,0 +1 @@
+@AGENTS.md
diff --git a/crates/oxc_formatter_tests/Cargo.toml b/crates/oxc_formatter_tests/Cargo.toml
new file mode 100644
index 0000000000..e9cc673fc6
--- /dev/null
+++ b/crates/oxc_formatter_tests/Cargo.toml
@@ -0,0 +1,26 @@
+[package]
+name = "oxc_formatter_tests"
+version = "0.62.0"
+authors.workspace = true
+categories.workspace = true
+edition.workspace = true
+homepage.workspace = true
+include = ["/src"]
+keywords.workspace = true
+license.workspace = true
+publish = false
+repository.workspace = true
+rust-version.workspace = true
+description.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+oxc_formatter_core = { workspace = true }
+
+serde_json = { workspace = true }
+
+[lib]
+doctest = false
+test = false
diff --git a/crates/oxc_formatter_core/src/test_support/codegen.rs b/crates/oxc_formatter_tests/src/codegen.rs
similarity index 100%
rename from crates/oxc_formatter_core/src/test_support/codegen.rs
rename to crates/oxc_formatter_tests/src/codegen.rs
diff --git a/crates/oxc_formatter_core/src/test_support/harness.rs b/crates/oxc_formatter_tests/src/harness.rs
similarity index 98%
rename from crates/oxc_formatter_core/src/test_support/harness.rs
rename to crates/oxc_formatter_tests/src/harness.rs
index 7c0a0d47be..ecfbb77e3b 100644
--- a/crates/oxc_formatter_core/src/test_support/harness.rs
+++ b/crates/oxc_formatter_tests/src/harness.rs
@@ -17,7 +17,9 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use crate::{CoreFormatOptions, FormatOptions, IndentStyle, IndentWidth, LineEnding, LineWidth};
+use oxc_formatter_core::{
+    CoreFormatOptions, FormatOptions, IndentStyle, IndentWidth, LineEnding, LineWidth,
+};
 
 /// A single `options.json` entry: a JSON object of per-test format options.
 pub type OptionSet = serde_json::Map<String, serde_json::Value>;
diff --git a/crates/oxc_formatter_core/src/test_support/mod.rs b/crates/oxc_formatter_tests/src/lib.rs
similarity index 56%
rename from crates/oxc_formatter_core/src/test_support/mod.rs
rename to crates/oxc_formatter_tests/src/lib.rs
index 6c2e1421cd..4918b39c32 100644
--- a/crates/oxc_formatter_core/src/test_support/mod.rs
+++ b/crates/oxc_formatter_tests/src/lib.rs
@@ -1,9 +1,11 @@
-//! Fixture-test infrastructure shared across formatter crates.
-//!
-//! Gated behind the `test_harness` Cargo feature. Split into:
+//! Test infrastructure shared across formatter crates.
 //!
 //! - `harness`: runtime that walks fixtures, drives format passes, and assembles snapshots.
 //! - `codegen`: build-script helper that emits `#[test]` functions for each fixture.
+//!
+//! Consumer crates use `codegen` from `build.rs` (via `[build-dependencies]`) and `harness`
+//! from `tests/fixtures/mod.rs` (via `[dev-dependencies]`). This crate depends only on
+//! `oxc_formatter_core`, never on language crates, so both directions stay cycle-free.
 
 mod codegen;
 mod harness;
diff --git a/crates/oxc_formatter_yaml/Cargo.toml b/crates/oxc_formatter_yaml/Cargo.toml
index 60d91466f4..67edf2f3f3 100644
--- a/crates/oxc_formatter_yaml/Cargo.toml
+++ b/crates/oxc_formatter_yaml/Cargo.toml
@@ -26,12 +26,12 @@ oxc_span = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 oxc_tasks_common = { workspace = true }
 pico-args = { workspace = true }
 
 [build-dependencies]
-oxc_formatter_core = { workspace = true, features = ["test_harness"] }
+oxc_formatter_tests = { workspace = true }
 
 [lib]
 doctest = false
diff --git a/crates/oxc_formatter_yaml/build.rs b/crates/oxc_formatter_yaml/build.rs
index 4a3e27bd7d..d63f2595de 100644
--- a/crates/oxc_formatter_yaml/build.rs
+++ b/crates/oxc_formatter_yaml/build.rs
@@ -3,7 +3,7 @@ use std::{
     path::{Path, PathBuf},
 };
 
-use oxc_formatter_core::test_support::{GenerateConfig, generate_tests};
+use oxc_formatter_tests::{GenerateConfig, generate_tests};
 
 fn main() {
     let out_dir = env::var("OUT_DIR").unwrap();
diff --git a/crates/oxc_formatter_yaml/tests/fixtures/mod.rs b/crates/oxc_formatter_yaml/tests/fixtures/mod.rs
index 7583d9958c..1f4998962e 100644
--- a/crates/oxc_formatter_yaml/tests/fixtures/mod.rs
+++ b/crates/oxc_formatter_yaml/tests/fixtures/mod.rs
@@ -1,9 +1,9 @@
 use std::path::Path;
 
 use oxc_allocator::Allocator;
-use oxc_formatter_core::{
-    LineEnding,
-    test_support::{FixtureFormatter, OptionSet, apply_core_options, build_fixture_snapshot},
+use oxc_formatter_core::LineEnding;
+use oxc_formatter_tests::{
+    FixtureFormatter, OptionSet, apply_core_options, build_fixture_snapshot,
 };
 use oxc_formatter_yaml::{ProseWrap, YamlFormatOptions, format};