diff --git a/.changeset/cli-specversion-probe.md b/.changeset/cli-specversion-probe.md
new file mode 100644
index 0000000000..732534b3f3
--- /dev/null
+++ b/.changeset/cli-specversion-probe.md
@@ -0,0 +1,6 @@
+---
+"@workflow/core": patch
+"@workflow/cli": patch
+---
+
+CLI `start` command probes deployment specVersion via health check before choosing queue transport. Health check always uses JSON transport for compatibility with old deployments.
diff --git a/packages/cli/src/lib/inspect/run.ts b/packages/cli/src/lib/inspect/run.ts
index a5f89366b3..335bb9544d 100644
--- a/packages/cli/src/lib/inspect/run.ts
+++ b/packages/cli/src/lib/inspect/run.ts
@@ -1,4 +1,5 @@
 import { start } from '@workflow/core/runtime';
+import { healthCheck } from '@workflow/core/runtime/helpers';
 import type { WorkflowRun, World } from '@workflow/world';
 import { logger } from '../config/log.js';
 
@@ -57,7 +58,27 @@ export const startRun = async (
   const deploymentId = run.deploymentId;
   const workflowId = await getWorkflowName(world, workflowNameOrRunId);
 
-  const newRun = await start({ workflowId }, jsonArgs, { deploymentId });
+  // Probe the deployment's specVersion via health check so we use the
+  // correct queue transport (JSON for old deployments, CBOR for new).
+  // Falls back to the run's specVersion if the health check fails
+  // (e.g. old deployment without health check support).
+  let specVersion = run.specVersion;
+  try {
+    const hc = await healthCheck(world, 'workflow', {
+      deploymentId,
+      timeout: 10_000,
+    });
+    if (hc.healthy && hc.specVersion != null) {
+      specVersion = hc.specVersion;
+    }
+  } catch {
+    // Health check failed — use run's specVersion as fallback
+  }
+
+  const newRun = await start({ workflowId }, jsonArgs, {
+    deploymentId,
+    specVersion,
+  });
 
   if (opts.json) {
     process.stdout.write(JSON.stringify(newRun, null, 2));
diff --git a/packages/core/e2e/e2e.test.ts b/packages/core/e2e/e2e.test.ts
index fc1c486a88..f1091dc050 100644
--- a/packages/core/e2e/e2e.test.ts
+++ b/packages/core/e2e/e2e.test.ts
@@ -1550,6 +1550,7 @@ describe('e2e', () => {
         healthy: true,
         endpoint: '/.well-known/workflow/v1/flow',
         specVersion: SPEC_VERSION_CURRENT,
+        workflowCoreVersion: expect.any(String),
       });
 
       // Test the step endpoint health check
@@ -1568,6 +1569,7 @@ describe('e2e', () => {
         healthy: true,
         endpoint: '/.well-known/workflow/v1/step',
         specVersion: SPEC_VERSION_CURRENT,
+        workflowCoreVersion: expect.any(String),
       });
     }
   );
diff --git a/packages/core/src/runtime/helpers.ts b/packages/core/src/runtime/helpers.ts
index fbabe993eb..b4597c6d07 100644
--- a/packages/core/src/runtime/helpers.ts
+++ b/packages/core/src/runtime/helpers.ts
@@ -4,12 +4,17 @@ import type {
   ValidQueueName,
   World,
 } from '@workflow/world';
-import { HealthCheckPayloadSchema, SPEC_VERSION_CURRENT } from '@workflow/world';
+import {
+  HealthCheckPayloadSchema,
+  SPEC_VERSION_CURRENT,
+  SPEC_VERSION_LEGACY,
+} from '@workflow/world';
 import { monotonicFactory } from 'ulid';
 
 import { runtimeLogger } from '../logger.js';
 import * as Attribute from '../telemetry/semantic-conventions.js';
 import { getSpanKind, trace } from '../telemetry.js';
+import { version as workflowCoreVersion } from '../version.js';
 import { getWorld } from './world.js';
 
 /** Default timeout for health checks in milliseconds */
@@ -99,6 +104,7 @@ export async function handleHealthCheckMessage(
     endpoint,
     correlationId: healthCheck.correlationId,
     specVersion: SPEC_VERSION_CURRENT,
+    workflowCoreVersion,
     timestamp: Date.now(),
   });
   // Use a fake runId that passes validation.
@@ -114,6 +120,8 @@ export type HealthCheckEndpoint = 'workflow' | 'step';
 export interface HealthCheckOptions {
   /** Timeout in milliseconds to wait for health check response. Default: 30000 (30s) */
   timeout?: number;
+  /** Deployment ID to send the health check to. Falls back to process.env.VERCEL_DEPLOYMENT_ID. */
+  deploymentId?: string;
 }
 
 /**
@@ -186,6 +194,12 @@ function parseHealthCheckResponse(
   try {
     response = JSON.parse(responseText);
   } catch {
+    // Old deployments (specVersion < 3) return plain text like
+    // 'Workflow SDK "..." endpoint is healthy'. Treat any non-empty
+    // text response as a healthy deployment with unknown specVersion.
+    if (responseText.length > 0) {
+      return { healthy: true };
+    }
     return null;
   }
 
@@ -225,10 +239,16 @@ export async function healthCheck(
   const startTime = Date.now();
 
   try {
-    await world.queue(queueName, {
-      __healthCheck: true,
-      correlationId,
-    });
+    await world.queue(
+      queueName,
+      { __healthCheck: true, correlationId },
+      {
+        // Use JSON transport so the health check works against both
+        // old (JSON-only) and new (dual) deployments.
+        specVersion: SPEC_VERSION_LEGACY,
+        deploymentId: options?.deploymentId,
+      }
+    );
 
     while (Date.now() - startTime < timeout) {
       try {
@@ -375,6 +395,7 @@ export function withHealthCheck(
           healthy: true,
           endpoint: url.pathname,
           specVersion: SPEC_VERSION_CURRENT,
+          workflowCoreVersion,
         }),
         {
           status: 200,