diff --git a/src/ModelContextProtocol.Core/Authentication/IdentityAssertionGrantProvider.cs b/src/ModelContextProtocol.Core/Authentication/IdentityAssertionGrantProvider.cs
index 41d2ea36d..ad5524812 100644
--- a/src/ModelContextProtocol.Core/Authentication/IdentityAssertionGrantProvider.cs
+++ b/src/ModelContextProtocol.Core/Authentication/IdentityAssertionGrantProvider.cs
@@ -169,6 +169,16 @@ private async Task<TokenContainer> AcquireAccessTokenAsync(
         var mcpAuthMetadata = await IdentityAssertionGrant.DiscoverAuthServerMetadataAsync(
             authorizationServerUrl, _httpClient, cancellationToken).ConfigureAwait(false);
 
+        // The issuer becomes the JAG audience below, so validate it first (RFC 8414 Section 3.3): it MUST be
+        // identical to the issuer identifier used for discovery. Like ClientOAuthProvider, compare the exact
+        // strings without normalizing case, trailing slashes or percent-encoding.
+        if (mcpAuthMetadata.Issuer is not null &&
+            !string.Equals(mcpAuthMetadata.Issuer.OriginalString, authorizationServerUrl.OriginalString, StringComparison.Ordinal))
+        {
+            throw new IdentityAssertionGrantException(
+                $"Authorization server metadata issuer '{mcpAuthMetadata.Issuer.OriginalString}' does not match the expected issuer '{authorizationServerUrl.OriginalString}' (RFC 8414 Section 3.3).");
+        }
+
         var mcpTokenEndpoint = mcpAuthMetadata.TokenEndpoint?.ToString()
             ?? throw new IdentityAssertionGrantException(
                 $"MCP authorization server metadata at {authorizationServerUrl} missing token_endpoint.");
@@ -196,7 +206,10 @@ private async Task<TokenContainer> AcquireAccessTokenAsync(
             new RequestJwtAuthGrantOptions
             {
                 TokenEndpoint = idpTokenEndpoint,
-                Audience = authorizationServerUrl.ToString(),
+                // The JAG audience is the MCP authorization server's issuer identifier (RFC 8414), not the
+                // caller-supplied URL, which can differ from it (e.g. Uri.ToString() appends a trailing slash).
+                // OriginalString preserves the advertised issuer exactly as published.
+                Audience = mcpAuthMetadata.Issuer?.OriginalString ?? authorizationServerUrl.ToString(),
                 Resource = resourceUrl.ToString(),
                 IdToken = idToken,
                 ClientId = _options.IdpClientId,
diff --git a/tests/ModelContextProtocol.Tests/IdentityAssertionGrantTests.cs b/tests/ModelContextProtocol.Tests/IdentityAssertionGrantTests.cs
index 92e81c93c..09e6109b4 100644
--- a/tests/ModelContextProtocol.Tests/IdentityAssertionGrantTests.cs
+++ b/tests/ModelContextProtocol.Tests/IdentityAssertionGrantTests.cs
@@ -95,6 +95,108 @@ public async Task IdentityAssertionGrantProvider_FullFlow_ReturnsAccessToken()
         Assert.Equal(3600, tokens.ExpiresIn);
     }
 
+    [Theory]
+    [InlineData("https://auth.example.com", "https://auth.example.com", "https://auth.example.com")]
+    [InlineData("https://auth.example.com/tenant", "https://auth.example.com/tenant", "https://auth.example.com/tenant")]
+    [InlineData("https://auth.example.com", null, "https://auth.example.com/")]
+    public async Task IdentityAssertionGrantProvider_UsesDiscoveredIssuerAsJagAudience(
+        string authorizationServerUrl, string? advertisedIssuer, string expectedAudience)
+    {
+        string? audience = null;
+        _mockHandler.AsyncHandler = async request =>
+        {
+            var url = request.RequestUri!.ToString();
+            if (url.Contains(".well-known"))
+            {
+                return JsonResponse(HttpStatusCode.OK, new JsonObject
+                {
+                    ["issuer"] = advertisedIssuer,
+                    ["token_endpoint"] = "https://auth.example.com/token",
+                });
+            }
+
+            if (url.Contains("idp.example.com"))
+            {
+                var body = await request.Content!.ReadAsStringAsync(TestContext.Current.CancellationToken);
+                audience = body.Split('&')
+                    .Select(pair => pair.Split('='))
+                    .Where(kv => kv[0] == "audience")
+                    .Select(kv => WebUtility.UrlDecode(kv[1]))
+                    .Single();
+
+                return JsonResponse(HttpStatusCode.OK, new JsonObject
+                {
+                    ["access_token"] = "mock-jag",
+                    ["issued_token_type"] = "urn:ietf:params:oauth:token-type:id-jag",
+                    ["token_type"] = "N_A",
+                });
+            }
+
+            return JsonResponse(HttpStatusCode.OK, new JsonObject
+            {
+                ["access_token"] = "final-access-token",
+                ["token_type"] = "Bearer",
+            });
+        };
+
+        var provider = new IdentityAssertionGrantProvider(
+            new IdentityAssertionGrantProviderOptions
+            {
+                ClientId = "mcp-client-id",
+                IdpTokenEndpoint = "https://idp.example.com/token",
+                IdpClientId = "idp-client-id",
+                IdTokenCallback = (_, _) => Task.FromResult("mock-id-token"),
+            },
+            _httpClient);
+
+        await provider.GetAccessTokenAsync(
+            new Uri("https://resource.example.com"),
+            new Uri(authorizationServerUrl),
+            TestContext.Current.CancellationToken);
+
+        Assert.Equal(expectedAudience, audience);
+    }
+
+    [Theory]
+    [InlineData("https://auth.example.com/", "https://auth.example.com/tenant")]
+    [InlineData("https://auth.example.com/", "https://auth.example.com")]
+    public async Task IdentityAssertionGrantProvider_RejectsMismatchedIssuer(
+        string authorizationServerUrl, string advertisedIssuer)
+    {
+        var idpCalled = false;
+        _mockHandler.Handler = request =>
+        {
+            if (request.RequestUri!.ToString().Contains("idp.example.com"))
+            {
+                idpCalled = true;
+            }
+
+            return JsonResponse(HttpStatusCode.OK, new JsonObject
+            {
+                ["issuer"] = advertisedIssuer,
+                ["token_endpoint"] = "https://auth.example.com/token",
+            });
+        };
+
+        var provider = new IdentityAssertionGrantProvider(
+            new IdentityAssertionGrantProviderOptions
+            {
+                ClientId = "mcp-client-id",
+                IdpTokenEndpoint = "https://idp.example.com/token",
+                IdpClientId = "idp-client-id",
+                IdTokenCallback = (_, _) => Task.FromResult("mock-id-token"),
+            },
+            _httpClient);
+
+        var ex = await Assert.ThrowsAsync<IdentityAssertionGrantException>(() => provider.GetAccessTokenAsync(
+            new Uri("https://resource.example.com"),
+            new Uri(authorizationServerUrl),
+            TestContext.Current.CancellationToken));
+
+        Assert.Contains("RFC 8414 Section 3.3", ex.Message);
+        Assert.False(idpCalled);
+    }
+
     [Fact]
     public Task IdentityAssertionGrantProvider_DefaultsToPostRegardlessOfMetadataOrder() =>
         AssertMcpTokenEndpointAuthenticationAsync(