diff --git a/src/runtime/process-io.ts b/src/runtime/process-io.ts
index 4178c8f..c87b275 100644
--- a/src/runtime/process-io.ts
+++ b/src/runtime/process-io.ts
@@ -676,7 +676,9 @@ export class ProcessIO extends BoundedContext implements Transport {
    */
   private handleStdinError(err: Error): void {
     // EPIPE means process died
-    const error = new BridgeProtocolError(`stdin error: ${err.message}`);
+    const error = new BridgeProtocolError(
+      this.withStderrTail(`stdin error: ${err.message}`)
+    );
 
     // Reject all pending writes
     for (const queued of this.writeQueue) {
@@ -697,7 +699,9 @@ export class ProcessIO extends BoundedContext implements Transport {
    * This can occur during pipe errors or when the process crashes.
    */
   private handleStdoutError(err: Error): void {
-    const error = new BridgeProtocolError(`stdout error: ${err.message}`);
+    const error = new BridgeProtocolError(
+      this.withStderrTail(`stdout error: ${err.message}`)
+    );
     this.rejectAllPending(error);
     this.markForRestart();
   }
@@ -708,7 +712,9 @@ export class ProcessIO extends BoundedContext implements Transport {
    */
   private handleStderrError(err: Error): void {
     // Stderr errors are less critical but still indicate process health issues
-    const error = new BridgeProtocolError(`stderr error: ${err.message}`);
+    const error = new BridgeProtocolError(
+      this.withStderrTail(`stderr error: ${err.message}`)
+    );
     this.rejectAllPending(error);
     this.markForRestart();
   }
@@ -763,7 +769,7 @@ export class ProcessIO extends BoundedContext implements Transport {
   private writeToStdin(data: string): Promise<void> {
     return new Promise<void>((resolve, reject) => {
       if (!this.process?.stdin || this.processExited) {
-        reject(new BridgeProtocolError('Process stdin not available'));
+        reject(new BridgeProtocolError(this.withStderrTail('Process stdin not available')));
         return;
       }
 
@@ -788,7 +794,8 @@ export class ProcessIO extends BoundedContext implements Transport {
       } catch (err) {
         // Synchronous write error (e.g., EPIPE)
         this.markForRestart();
-        reject(new BridgeProtocolError(`Write error: ${err instanceof Error ? err.message : 'unknown'}`));
+        const errorMessage = err instanceof Error ? err.message : 'unknown';
+        reject(new BridgeProtocolError(this.withStderrTail(`Write error: ${errorMessage}`)));
       }
     });
   }
@@ -804,7 +811,7 @@ export class ProcessIO extends BoundedContext implements Transport {
         // Process died - reject all queued writes
         for (const q of this.writeQueue) {
           this.clearQueuedWriteTimeout(q);
-          q.reject(new BridgeProtocolError('Process stdin not available'));
+          q.reject(new BridgeProtocolError(this.withStderrTail('Process stdin not available')));
         }
         this.writeQueue.length = 0;
         this.markForRestart();
@@ -841,8 +848,9 @@ export class ProcessIO extends BoundedContext implements Transport {
         }
       } catch (err) {
         // Synchronous write error (e.g., EPIPE) - reject this and all remaining writes
+        const errorMessage = err instanceof Error ? err.message : 'unknown';
         const error = new BridgeProtocolError(
-          `Write error: ${err instanceof Error ? err.message : 'unknown'}`
+          this.withStderrTail(`Write error: ${errorMessage}`)
         );
         queued.reject(error);
         for (const q of this.writeQueue) {
@@ -867,6 +875,14 @@ export class ProcessIO extends BoundedContext implements Transport {
     return this.stderrBuffer.trim();
   }
 
+  /**
+   * Append stderr context when available.
+   */
+  private withStderrTail(message: string): string {
+    const stderrTail = this.getStderrTail();
+    return stderrTail ? `${message}. Stderr:\n${stderrTail}` : message;
+  }
+
   /**
    * Handle a protocol error by rejecting all pending requests.
    */
diff --git a/test/adversarial_playground.test.ts b/test/adversarial_playground.test.ts
index 560dbc6..98e7edf 100644
--- a/test/adversarial_playground.test.ts
+++ b/test/adversarial_playground.test.ts
@@ -157,6 +157,25 @@ describeAdversarial('Adversarial playground', () => {
     testTimeoutMs
   );
 
+  it(
+    'surfaces invalid TYWRAP_CODEC_MAX_BYTES as an explicit startup error',
+    async () => {
+      const bridge = await createBridge({
+        env: { TYWRAP_CODEC_MAX_BYTES: 'not-a-number' },
+      });
+      if (!bridge) return;
+
+      try {
+        await expect(callAdversarial(bridge, 'echo', ['value'])).rejects.toThrow(
+          /TYWRAP_CODEC_MAX_BYTES/
+        );
+      } finally {
+        await bridge.dispose();
+      }
+    },
+    testTimeoutMs
+  );
+
   it(
     'rejects requests that exceed TYWRAP_REQUEST_MAX_BYTES',
     async () => {
diff --git a/test/transport.test.ts b/test/transport.test.ts
index 92ac458..c8e96c6 100644
--- a/test/transport.test.ts
+++ b/test/transport.test.ts
@@ -404,6 +404,7 @@ describe('ProcessIO', () => {
   interface ProcessIOInternals {
     _state: string;
     processExited: boolean;
+    stderrBuffer: string;
     process: { stdin: { write: (data: string) => boolean } } | null;
     handleStdinDrain: () => void;
     handleResponseLine: (line: string) => void;
@@ -583,6 +584,26 @@ describe('ProcessIO', () => {
       await expect(firstPending).resolves.toContain(`"id":${firstId}`);
       await expect(secondPending).resolves.toContain(`"id":${secondId}`);
     });
+
+    it('includes stderr diagnostics when stdin write fails', async () => {
+      const transport = new ProcessIO({ bridgeScript: '/path/to/bridge.py' });
+
+      const internals = transport as unknown as ProcessIOInternals;
+      internals._state = 'ready';
+      internals.processExited = false;
+      internals.stderrBuffer =
+        'CodecMaxBytesParseError: TYWRAP_CODEC_MAX_BYTES must be an integer byte count';
+      internals.process = {
+        stdin: {
+          write: (): boolean => {
+            throw new Error('write EPIPE');
+          },
+        },
+      };
+
+      const message = JSON.stringify(createValidMessage({ id: 303 }));
+      await expect(transport.send(message, 1000)).rejects.toThrow(/TYWRAP_CODEC_MAX_BYTES/);
+    });
   });
 
   describe('abstract method stubs', () => {