diff --git a/packages/client/src/client/auth.ts b/packages/client/src/client/auth.ts
index 1a021be18..0391e35e9 100644
--- a/packages/client/src/client/auth.ts
+++ b/packages/client/src/client/auth.ts
@@ -1439,6 +1439,10 @@ export async function executeTokenRequest(
         applyClientAuthentication(authMethod, clientInformation as OAuthClientInformation, headers, tokenRequestParams);
     }
 
+    // Ensure Content-Type is always form-urlencoded for the token endpoint (OAuth 2.1 §3.2).
+    // Some addClientAuthentication implementations may have inadvertently set a different value.
+    headers.set('Content-Type', 'application/x-www-form-urlencoded');
+
     const response = await (fetchFn ?? fetch)(tokenUrl, {
         method: 'POST',
         headers,
diff --git a/packages/client/src/client/streamableHttp.ts b/packages/client/src/client/streamableHttp.ts
index 3d45b60e9..20f294a22 100644
--- a/packages/client/src/client/streamableHttp.ts
+++ b/packages/client/src/client/streamableHttp.ts
@@ -578,6 +578,17 @@ export class StreamableHTTPClientTransport implements Transport {
                     }
                 }
 
+                // Per MCP spec §Session Management: on 404 with an Mcp-Session-Id
+                // header, the client MUST clear the session ID and re-initialize.
+                // Clear the stale session so subsequent calls don't keep failing.
+                if (response.status === 404) {
+                    this._sessionId = undefined;
+                    throw new SdkError(SdkErrorCode.ClientHttpNotFound, `Session not found (HTTP 404). Clear and re-initialize. ${text}`.trim(), {
+                        status: 404,
+                        text
+                    });
+                }
+
                 throw new SdkError(SdkErrorCode.ClientHttpNotImplemented, `Error POSTing to endpoint: ${text}`, {
                     status: response.status,
                     text
diff --git a/packages/core/src/errors/sdkErrors.ts b/packages/core/src/errors/sdkErrors.ts
index f53c07ccf..44cb78235 100644
--- a/packages/core/src/errors/sdkErrors.ts
+++ b/packages/core/src/errors/sdkErrors.ts
@@ -31,6 +31,7 @@ export enum SdkErrorCode {
     ClientHttpNotImplemented = 'CLIENT_HTTP_NOT_IMPLEMENTED',
     ClientHttpAuthentication = 'CLIENT_HTTP_AUTHENTICATION',
     ClientHttpForbidden = 'CLIENT_HTTP_FORBIDDEN',
+    ClientHttpNotFound = 'CLIENT_HTTP_NOT_FOUND',
     ClientHttpUnexpectedContent = 'CLIENT_HTTP_UNEXPECTED_CONTENT',
     ClientHttpFailedToOpenStream = 'CLIENT_HTTP_FAILED_TO_OPEN_STREAM',
     ClientHttpFailedToTerminateSession = 'CLIENT_HTTP_FAILED_TO_TERMINATE_SESSION'