diff --git a/src/handlers/keys.tsx b/src/handlers/keys.tsx
index 295d0ac93..ff9740076 100644
--- a/src/handlers/keys.tsx
+++ b/src/handlers/keys.tsx
@@ -11,6 +11,13 @@ import { contextKey, globalFlag } from "../router";
 // withRegion middleware can fall back to AWS_REGION / the shared config file.
 export const RegionKey = globalFlag("region", "AWS region", z.string().optional());
 
+// withRegion overwrites RegionKey with the fully-resolved region (flag -> env ->
+// config -> DEFAULT_REGION) before any handler runs, so by the time a leaf reads
+// ctx.value(RegionKey) it's never undefined. This key preserves whether --region
+// was actually passed, for callers (like the v0 compat path) that must tell
+// "user asked for a region" apart from "we picked one for them".
+export const RegionExplicitKey = contextKey<boolean>("region.explicit");
+
 export const DebugKey = globalFlag("debug", "debug logging", z.boolean().default(false));
 
 export const JsonKey = globalFlag("json", "JSON output", z.boolean().default(false));
diff --git a/src/handlers/project/deploy/index.test.ts b/src/handlers/project/deploy/index.test.ts
index 7694de677..c1936fb5d 100644
--- a/src/handlers/project/deploy/index.test.ts
+++ b/src/handlers/project/deploy/index.test.ts
@@ -273,10 +273,13 @@ describe("project deploy handler", () => {
         },
       ]);
       expect(subject.io.stdout()).toBe("");
+      const regionWarning =
+        "--region is ignored for v0 projects; v0 uses the target's region from " +
+        "aws-targets.json, or AWS_REGION / your AWS profile.";
       expect(subject.io.stderr()).toBe(
         flag === "--json"
-          ? ""
-          : "--region is ignored for v0 projects; v0 resolves region from aws-targets.json.\n" +
+          ? regionWarning
+          : `${regionWarning}\n` +
               "This is a v0 project; running the latest v0 AgentCore CLI deploy in compatibility mode.",
       );
       expect(
@@ -285,6 +288,31 @@ describe("project deploy handler", () => {
     },
   );
 
+  test("does not warn about --region when it was never passed", async () => {
+    const projectRoot = await inProjectWithTargets("[]");
+    const specPath = join(projectRoot, "agentcore", "agentcore.json");
+    const manifestPath = join(projectRoot, "agentcore", "cdk", "package.json");
+    await writeFile(specPath, JSON.stringify({ name: "orders", version: 1 }));
+    await writeFile(
+      manifestPath,
+      JSON.stringify({ dependencies: { "@aws/agentcore-cdk": "^0.1.0-alpha.19" } }),
+    );
+    const legacyDeploys: V0DeployInput[] = [];
+    const subject = await testDeployCommand({ outputs: {} }, [], {
+      runV0Deploy: async (input) => {
+        legacyDeploys.push(input);
+        return 0;
+      },
+    });
+
+    await subject.run(["--yes"]);
+
+    expect(legacyDeploys).toHaveLength(1);
+    expect(subject.io.stderr()).toBe(
+      "This is a v0 project; running the latest v0 AgentCore CLI deploy in compatibility mode.",
+    );
+  });
+
   test("preserves the v0 deployer's non-zero exit code", async () => {
     await inProjectWithTargets();
     await writeFile(
diff --git a/src/handlers/project/deploy/index.ts b/src/handlers/project/deploy/index.ts
index 033473ac0..0ae642447 100644
--- a/src/handlers/project/deploy/index.ts
+++ b/src/handlers/project/deploy/index.ts
@@ -17,7 +17,7 @@ import {
 } from "../../../router";
 import { JsonRendererKey } from "../../../tui";
 import { runWithProgress } from "../../../tui/progress";
-import { JsonKey, RegionKey } from "../../keys";
+import { JsonKey, RegionExplicitKey, RegionKey } from "../../keys";
 import { renderJsonError } from "../../utils";
 import type { DeployResult, Project, ProjectManager, TeardownConfirmationHandler } from "../types";
 
@@ -81,8 +81,7 @@ export const createDeployProjectHandler = (config: DeployProjectHandlerConfig) =
       recordProjectResourceCounts(ctx, project);
       const jsonOutput = ctx.require(JsonKey);
       if (project.legacy) {
-        warnIfRegionIgnored(config, ctx, jsonOutput);
-        await delegateV0Deploy(config, {
+        await delegateV0Deploy(config, ctx, {
           projectRoot: project.rootPath,
           target: flags.target,
           yes: flags.yes,
@@ -193,13 +192,11 @@ function withV0Deploy(config: DeployProjectHandlerConfig): Middleware {
     handle: async (ctx, flags, args) => {
       const projectRoot = enclosingProjectRoot(process.cwd());
       if (projectRoot && (await isV0Project(projectRoot))) {
-        const jsonOutput = ctx.require(JsonKey);
-        warnIfRegionIgnored(config, ctx, jsonOutput);
-        await delegateV0Deploy(config, {
+        await delegateV0Deploy(config, ctx, {
           projectRoot,
           target: flags.target as string,
           yes: flags.yes as boolean,
-          json: jsonOutput,
+          json: ctx.require(JsonKey),
         });
         return;
       }
@@ -208,30 +205,27 @@ function withV0Deploy(config: DeployProjectHandlerConfig): Middleware {
   });
 }
 
-// --region is a v1 global flag that still resolves (and defaults) for every
-// command, but v0 deploys have no such flag -- they resolve region entirely
-// from aws-targets.json, with env/profile as a fallback. An explicitly passed
-// --region is otherwise silently discarded on this path, trading what used to
-// be a loud "unknown option" failure for an invisible no-op.
-function warnIfRegionIgnored(
-  config: DeployProjectHandlerConfig,
-  ctx: Context,
-  jsonOutput: boolean,
-): void {
-  if (ctx.value(RegionKey) !== undefined && !jsonOutput) {
-    config.io.stderr.write(
-      "--region is ignored for v0 projects; v0 resolves region from aws-targets.json.\n",
-    );
-  }
-}
-
 async function delegateV0Deploy(
   config: DeployProjectHandlerConfig,
+  ctx: Context,
   input: Parameters<V0DeployRunner>[0],
 ): Promise<void> {
   // A migration stopped while writing the files leaves a v0 spec beside a v1 CDK
   // app; the v0 deployer mustn't deploy that.
   await assertNoPendingMigration(input.projectRoot, input.target);
+  // --region is a v1 global flag that still resolves (and defaults) for every
+  // command, but v0 deploys have no such flag -- they resolve region entirely
+  // from aws-targets.json's saved target, falling back to AWS_REGION or the AWS
+  // profile. An explicitly passed --region is otherwise silently discarded on
+  // this path, trading what used to be a loud "unknown option" failure for an
+  // invisible no-op; warn regardless of --json since stderr is informational,
+  // not part of the JSON contract.
+  if (ctx.value(RegionExplicitKey)) {
+    config.io.stderr.write(
+      "--region is ignored for v0 projects; v0 uses the target's region from " +
+        "aws-targets.json, or AWS_REGION / your AWS profile.\n",
+    );
+  }
   if (!input.json) {
     config.io.stderr.write(
       "This is a v0 project; running the latest v0 AgentCore CLI deploy in compatibility mode.\n",
diff --git a/src/handlers/project/dev/index.test.ts b/src/handlers/project/dev/index.test.ts
index d60063eee..0fe7088dd 100644
--- a/src/handlers/project/dev/index.test.ts
+++ b/src/handlers/project/dev/index.test.ts
@@ -715,7 +715,9 @@ describe("project dev harnesses", () => {
       },
     ]);
     expect(subject.deployments).toEqual([]);
-    expect(subject.io.stderr()).toContain("running the latest v0 AgentCore CLI deploy");
+    expect(subject.io.stderr()).toBe(
+      "This is a v0 project; running the latest v0 AgentCore CLI deploy in compatibility mode.",
+    );
   });
 
   test("harness dev's deploy refuses a target a migration hasn't finished", async () => {
diff --git a/src/middleware/withRegion.tsx b/src/middleware/withRegion.tsx
index eb305ac3b..e63a75480 100644
--- a/src/middleware/withRegion.tsx
+++ b/src/middleware/withRegion.tsx
@@ -1,10 +1,12 @@
-import { RegionKey } from "../handlers/keys";
+import { RegionExplicitKey, RegionKey } from "../handlers/keys";
 import { resolveRegion } from "../core/region";
 import { type Middleware } from "../router";
 
 // withRegion resolves the effective AWS region and pins it onto the context under
 // RegionKey so the whole app uses a single, consistent value. There is always a
-// region: it falls back to DEFAULT_REGION when nothing else is configured.
+// region: it falls back to DEFAULT_REGION when nothing else is configured. It also
+// records, under RegionExplicitKey, whether the raw --region flag was present
+// before this resolution -- the only place that distinction is still observable.
 export function withRegion(): Middleware {
   return (h) => ({
     name: () => h.name(),
@@ -14,8 +16,13 @@ export function withRegion(): Middleware {
     doesSupportTui: () => h.doesSupportTui(),
     children: () => h.children(),
     handle: async (ctx, flags, args) => {
+      const explicit = ctx.value(RegionKey) !== undefined;
       const region = await resolveRegion(ctx.value(RegionKey));
-      await h.handle(ctx.withValue(RegionKey, region), flags, args);
+      await h.handle(
+        ctx.withValue(RegionKey, region).withValue(RegionExplicitKey, explicit),
+        flags,
+        args,
+      );
     },
   });
 }