diff --git a/CHANGELOG.md b/CHANGELOG.md
index 940aa6893..82d25a32f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -72,6 +72,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Bug Fixes
 
+* **proxy/auth:** classify real Anthropic OAuth tokens correctly. `classify_auth_mode` matched OAuth on the `sk-ant-oat-` prefix, but real access tokens are `sk-ant-oat01-...` (a version number, no dash after `oat`), so every real subscription/OAuth token fell through to the `sk-` branch and was tagged `PAYG` — enabling aggressive lossy compression, auto `cache_control`, and `prompt_cache_key` injection on subscription-bound requests the classifier is meant to route to the passthrough-prefer path. The prefix is now the dash-less `sk-ant-oat` (still matches the legacy dashed shape). The existing parity tests only passed because they used a synthetic `sk-ant-oat-01-` fixture; a regression test now covers the real `sk-ant-oat01-` format.
 * **install:** stop leaking a file descriptor on every `headroom install start`. `start_detached_agent()` opened the agent log file and handed it to `subprocess.Popen` but never closed the parent's copy, so each call leaked one fd (and pinned the log file open against rotation). The parent now closes its copy in a `try/finally` once the child has inherited it — the close also runs if `Popen` raises ([#1554](https://github.com/headroomlabs-ai/headroom/issues/1554)).
 * **proxy:** include the system prompt, tools, and the response-shaping request fields in the SemanticCache key. `_compute_key` hashed only `{model, messages}`, so two non-streaming requests with identical messages but a different top-level `system` prompt, tool set, sampling config, or output-shaping field collided on one key and the second caller was served the first's cached response — generated under different request semantics, in the default config (`cache_enabled` defaults on). The key now folds the request fields that shape generation — `temperature`/`top_p`/`top_k`/`max_tokens`/`stop`, plus OpenAI `tool_choice`/`response_format`/`parallel_tool_calls`/`seed`/`presence_penalty`/`frequency_penalty`/`logit_bias`/`n`/`logprobs`/`top_logprobs`/`reasoning_effort`/`verbosity`/`modalities` and Anthropic `thinking`/`tool_choice`/`output_config` — canonicalizing `system`/`tools` so a moved `cache_control` breakpoint does not fragment it, and the handlers snapshot the fields once at the cache read and reuse them at write so a body mutated by the pipeline cannot diverge the key. Non-streaming path only.
 * **learn (verbosity):** `--verbosity --apply --all` now aggregates the savings baseline across every project instead of overwriting it per project (last-project-wins), which previously left the output shaper with a tiny, unrepresentative baseline. The applied verbosity level comes from the project with the most samples ([#1288](https://github.com/headroomlabs-ai/headroom/pull/1288)).
diff --git a/crates/headroom-core/src/auth_mode.rs b/crates/headroom-core/src/auth_mode.rs
index 4f0242d9e..0c61087b5 100644
--- a/crates/headroom-core/src/auth_mode.rs
+++ b/crates/headroom-core/src/auth_mode.rs
@@ -93,10 +93,10 @@ const SUBSCRIPTION_UA_PREFIXES: &[&str] = &[
 /// 1. **Subscription UA prefix** → [`AuthMode::Subscription`].
 ///    The CLI's own auth-mode wins over the bearer token shape it
 ///    happens to be carrying — a Claude Code session uses a
-///    `sk-ant-oat-*` token but is a subscription client, not OAuth.
-/// 2. **`Authorization: Bearer sk-ant-oat-*`** → [`AuthMode::OAuth`]
+///    `sk-ant-oat*` token but is a subscription client, not OAuth.
+/// 2. **`Authorization: Bearer sk-ant-oat*`** → [`AuthMode::OAuth`]
 ///    (Claude Pro / Max OAuth). Checked before the broader `sk-` PAYG
-///    rule because `sk-ant-oat-` shares the `sk-` prefix.
+///    rule because `sk-ant-oat` shares the `sk-` prefix.
 /// 3. **`Authorization: Bearer sk-ant-api*` or `Bearer sk-*`** →
 ///    [`AuthMode::Payg`] (Anthropic / OpenAI API key).
 /// 4. **`Authorization: Bearer <jwt>`** (3 dot-separated segments) →
@@ -162,10 +162,11 @@ pub fn classify(headers: &HeaderMap) -> AuthMode {
     };
 
     if let Some(token) = auth.strip_prefix("Bearer ") {
-        // Order matters: the OAuth shape `sk-ant-oat-*` shares a
+        // Order matters: the OAuth shape `sk-ant-oat*` shares a
         // prefix with `sk-ant-api*` only at `sk-ant-`, so we check
-        // the OAuth shape FIRST. Then the broad PAYG shapes.
-        if token.starts_with("sk-ant-oat-") {
+        // the OAuth shape FIRST. Real OAuth access tokens are
+        // `sk-ant-oat01-...` (version number, no dash after `oat`).
+        if token.starts_with("sk-ant-oat") {
             return AuthMode::OAuth;
         }
         if token.starts_with("sk-ant-api") || token.starts_with("sk-") {
diff --git a/crates/headroom-core/tests/auth_mode.rs b/crates/headroom-core/tests/auth_mode.rs
index 461e41555..f0de13301 100644
--- a/crates/headroom-core/tests/auth_mode.rs
+++ b/crates/headroom-core/tests/auth_mode.rs
@@ -43,11 +43,19 @@ fn oauth_jwt_classified_oauth() {
 
 #[test]
 fn oauth_sk_ant_oat_classified_oauth() {
-    // Claude Pro / Max OAuth: `Bearer sk-ant-oat-...`.
+    // Legacy/synthetic Claude Pro / Max OAuth fixture.
     let h = headers(&[("authorization", "Bearer sk-ant-oat-01-abc123def456")]);
     assert_eq!(classify(&h), AuthMode::OAuth);
 }
 
+#[test]
+fn oauth_real_sk_ant_oat01_classified_oauth() {
+    // Real Anthropic OAuth access tokens are `sk-ant-oat01-...`:
+    // a version number, no dash after `oat`.
+    let h = headers(&[("authorization", "Bearer sk-ant-oat01-abc123def456")]);
+    assert_eq!(classify(&h), AuthMode::OAuth);
+}
+
 #[test]
 fn claude_code_ua_classified_subscription() {
     // Claude Code CLI: `User-Agent: claude-code/1.2.3 ...`.
diff --git a/headroom/proxy/auth_mode.py b/headroom/proxy/auth_mode.py
index 220ac4397..4999ac6f7 100644
--- a/headroom/proxy/auth_mode.py
+++ b/headroom/proxy/auth_mode.py
@@ -116,10 +116,10 @@ def classify_auth_mode(headers: Mapping[str, Any] | Any) -> AuthMode:
     1. **Subscription UA prefix** → :data:`AuthMode.SUBSCRIPTION`.
        The CLI's own auth-mode wins over the bearer token shape it
        happens to be carrying — a Claude Code session uses a
-       ``sk-ant-oat-*`` token but is a subscription client, not OAuth.
-    2. **``Authorization: Bearer sk-ant-oat-*``** → :data:`AuthMode.OAUTH`
+       ``sk-ant-oat*`` token but is a subscription client, not OAuth.
+    2. **``Authorization: Bearer sk-ant-oat*``** → :data:`AuthMode.OAUTH`
        (Claude Pro / Max OAuth). Checked before the broader ``sk-``
-       PAYG rule because ``sk-ant-oat-`` shares the ``sk-`` prefix.
+       PAYG rule because ``sk-ant-oat`` shares the ``sk-`` prefix.
     3. **``Authorization: Bearer sk-ant-api*`` or ``Bearer sk-*``** →
        :data:`AuthMode.PAYG` (Anthropic / OpenAI API key).
     4. **``Authorization: Bearer <jwt>``** (3 dot-separated segments)
@@ -151,9 +151,13 @@ def classify_auth_mode(headers: Mapping[str, Any] | Any) -> AuthMode:
 
     if auth.startswith("Bearer "):
         token = auth[len("Bearer ") :]
-        # Order matters: `sk-ant-oat-*` shares a prefix with
+        # Order matters: `sk-ant-oat*` shares a prefix with
         # `sk-ant-api*` only at `sk-ant-`, so check OAuth first.
-        if token.startswith("sk-ant-oat-"):
+        # Real Anthropic OAuth access tokens are `sk-ant-oat01-...`
+        # (a version number, no dash after `oat`), so match on the
+        # dash-less `sk-ant-oat` prefix — matching on `sk-ant-oat-`
+        # missed every real token and let it fall through to PAYG.
+        if token.startswith("sk-ant-oat"):
             return AuthMode.OAUTH
         if token.startswith("sk-ant-api") or token.startswith("sk-"):
             return AuthMode.PAYG
diff --git a/tests/test_auth_mode.py b/tests/test_auth_mode.py
index b4627bb89..874a923b9 100644
--- a/tests/test_auth_mode.py
+++ b/tests/test_auth_mode.py
@@ -47,6 +47,15 @@ def test_oauth_sk_ant_oat_classified_oauth() -> None:
     assert classify_auth_mode(headers) is AuthMode.OAUTH
 
 
+def test_oauth_real_sk_ant_oat01_classified_oauth() -> None:
+    """Real Anthropic OAuth access tokens are ``sk-ant-oat01-...`` (a version
+    number, no dash after ``oat``). These must classify as OAUTH — matching on
+    ``sk-ant-oat-`` missed them and let them fall through to PAYG, enabling
+    aggressive lossy compression on subscription-bound requests."""
+    headers = {"authorization": "Bearer sk-ant-oat01-abc123def456"}
+    assert classify_auth_mode(headers) is AuthMode.OAUTH
+
+
 def test_claude_code_ua_classified_subscription() -> None:
     """Claude Code CLI: ``User-Agent: claude-code/1.2.3 ...``."""
     headers = {"user-agent": "claude-code/1.2.3 (darwin; arm64)"}