Raise error when http_client and ssrf_safe=True are both provided

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https://claude.ai/code/session_012QKWmKd21vypDmxWbwuE4e

docs/servers/auth/token-verification.mdx

@@ -364,6 +364,10 @@ verifier = JWTVerifier(
 )
 ```
 
+<Warning>
+`JWTVerifier` does not support `http_client` when `ssrf_safe=True`. SSRF-safe mode requires a hardened transport that validates DNS resolution and connection targets, which cannot be guaranteed with a user-provided client. Attempting to use both will raise a `ValueError`.
+</Warning>
+
 <Note>
 When you provide an `http_client`, you are responsible for its lifecycle. The verifier will not close it. Use the server's `lifespan` to manage client cleanup:
 

src/fastmcp/server/auth/providers/jwt.py

@@ -189,10 +189,11 @@ class JWTVerifier(TokenVerifier):
             http_client: Optional httpx.AsyncClient for connection pooling. When provided,
                 the client is reused for JWKS fetches and the caller is responsible for
                 its lifecycle. When None (default), a fresh client is created per fetch.
-                Only used when ssrf_safe is False; SSRF-safe fetches use their own transport.
+                Cannot be used with ssrf_safe=True.
 
         Raises:
-            ValueError: If neither or both of `public_key` and `jwks_uri` are provided, or if `algorithm` is unsupported.
+            ValueError: If neither or both of `public_key` and `jwks_uri` are provided,
+                if `algorithm` is unsupported, or if `http_client` is provided with `ssrf_safe=True`.
         """
         if not public_key and not jwks_uri:
             raise ValueError("Either public_key or jwks_uri must be provided")
@@ -200,6 +201,12 @@ class JWTVerifier(TokenVerifier):
         if public_key and jwks_uri:
             raise ValueError("Provide either public_key or jwks_uri, not both")
 
+        if ssrf_safe and http_client is not None:
+            raise ValueError(
+                "http_client cannot be used with ssrf_safe=True; "
+                "SSRF-safe mode requires its own hardened transport"
+            )
+
         algorithm = algorithm or "RS256"
         if algorithm not in {
             "HS256",

tests/server/auth/providers/test_http_client.py

@@ -180,21 +180,17 @@ class TestJWTVerifierHttpClient:
         assert result is not None
         assert not shared_client.is_closed
 
-    async def test_ssrf_safe_ignores_http_client(
+    def test_ssrf_safe_rejects_http_client(
         self,
         shared_client: httpx.AsyncClient,
     ):
-        """When ssrf_safe=True, the custom http_client should NOT be used."""
-        verifier = JWTVerifier(
-            jwks_uri="https://auth.example.com/.well-known/jwks.json",
-            ssrf_safe=True,
-            http_client=shared_client,
-        )
-
-        # ssrf_safe uses ssrf_safe_fetch instead of httpx.AsyncClient
-        # The http_client is stored but not used in this code path
-        assert verifier._http_client is shared_client
-        assert verifier.ssrf_safe is True
+        """ssrf_safe=True and http_client cannot be used together."""
+        with pytest.raises(ValueError, match="cannot be used with ssrf_safe=True"):
+            JWTVerifier(
+                jwks_uri="https://auth.example.com/.well-known/jwks.json",
+                ssrf_safe=True,
+                http_client=shared_client,
+            )
 
 
 class TestGitHubHttpClient: