diff --git a/packages/loopover-engine/src/idea-intake.ts b/packages/loopover-engine/src/idea-intake.ts
index 644b902b6..ba0e89820 100644
--- a/packages/loopover-engine/src/idea-intake.ts
+++ b/packages/loopover-engine/src/idea-intake.ts
@@ -85,6 +85,10 @@ function isNonEmptyString(value: unknown): value is string {
   return typeof value === "string" && value.trim().length > 0;
 }
 
+// The "owner/name" slug check shared by both existing-repo input shapes (bare string and
+// `{ kind: "existing", repo }`): each segment must be a GitHub-legal slug.
+const TARGET_REPO_SLUG_PATTERN = /^[A-Za-z0-9._-]+\/[A-Za-z0-9._-]+$/;
+
 /** Validate + normalize a raw renter submission (spec §1). Returns every failure at once (never folds with
  *  `??`/`||`) so a caller can surface all problems in one pass rather than one-at-a-time. */
 export function validateIdeaSubmission(raw: unknown): IdeaValidationResult {
@@ -98,10 +102,17 @@ export function validateIdeaSubmission(raw: unknown): IdeaValidationResult {
   else if (input.body.length > IDEA_BODY_MAX_CHARS) errors.push("body_too_long");
   // Back-compat wire form: a bare "owner/name" string means an existing repo (each segment a GitHub-legal
   // slug -- an uninstallable/malformed repo is rejected at intake, never scored, since it can never produce a
-  // `go`). A `{ kind: "provision" }` object requests a not-yet-created repo (#7589). Anything else is missing.
+  // `go`). A canonical `{ kind: "existing", repo }` object — the exact shape this validator itself returns,
+  // and what a TypeScript caller building against the exported `IdeaTarget` union constructs — resolves the
+  // same way, with the same slug check, so `validateIdeaSubmission` round-trips its own output (#9609). A
+  // `{ kind: "provision" }` object requests a not-yet-created repo (#7589). Anything else is missing.
   let resolvedTarget: IdeaTarget | undefined;
   if (isNonEmptyString(input.targetRepo)) {
-    if (/^[A-Za-z0-9._-]+\/[A-Za-z0-9._-]+$/.test(input.targetRepo)) resolvedTarget = { kind: "existing", repo: input.targetRepo };
+    if (TARGET_REPO_SLUG_PATTERN.test(input.targetRepo)) resolvedTarget = { kind: "existing", repo: input.targetRepo };
+    else errors.push("target_repo_malformed");
+  } else if (typeof input.targetRepo === "object" && input.targetRepo !== null && (input.targetRepo as Record<string, unknown>).kind === "existing") {
+    const repo = (input.targetRepo as Record<string, unknown>).repo;
+    if (typeof repo === "string" && TARGET_REPO_SLUG_PATTERN.test(repo)) resolvedTarget = { kind: "existing", repo };
     else errors.push("target_repo_malformed");
   } else if (typeof input.targetRepo === "object" && input.targetRepo !== null && (input.targetRepo as Record<string, unknown>).kind === "provision") {
     resolvedTarget = { kind: "provision" };
diff --git a/test/unit/idea-intake-bridge.test.ts b/test/unit/idea-intake-bridge.test.ts
index 4d5d84f37..75f9fcfda 100644
--- a/test/unit/idea-intake-bridge.test.ts
+++ b/test/unit/idea-intake-bridge.test.ts
@@ -74,11 +74,42 @@ describe("validateIdeaSubmission", () => {
     if (provision.ok) expect(provision.idea.targetRepo).toEqual({ kind: "provision" });
   });
 
-  it("rejects an object targetRepo that is not a provision request", () => {
-    for (const targetRepo of [{}, { kind: "existing" }]) {
+  it("rejects an object targetRepo whose kind is neither existing nor provision", () => {
+    for (const targetRepo of [{}, { kind: "bogus" }]) {
       const r = validateIdeaSubmission(rawIdea({ targetRepo }));
       expect(r.ok).toBe(false);
-      if (!r.ok) expect(r.errors).toContain("target_repo_required");
+      if (!r.ok) expect(r.errors).toEqual(["target_repo_required"]);
+    }
+  });
+
+  it("accepts the canonical { kind: 'existing', repo } object the validator itself returns (#9609)", () => {
+    const r = validateIdeaSubmission(rawIdea({ targetRepo: { kind: "existing", repo: "acme/widgets" } }));
+    expect(r.ok).toBe(true);
+    if (r.ok) expect(r.idea.targetRepo).toEqual({ kind: "existing", repo: "acme/widgets" });
+  });
+
+  it("round-trips its own output: re-validating a validated idea succeeds with targetRepo unchanged (#9609)", () => {
+    for (const targetRepo of ["acme/widgets", { kind: "provision" }]) {
+      const first = validateIdeaSubmission(rawIdea({ targetRepo }));
+      expect(first.ok).toBe(true);
+      if (!first.ok) continue;
+      const second = validateIdeaSubmission(first.idea);
+      expect(second.ok).toBe(true);
+      if (second.ok) expect(second.idea.targetRepo).toEqual(first.idea.targetRepo);
+    }
+  });
+
+  it("flags a recognised-but-malformed { kind: 'existing' } repo as target_repo_malformed (#9609)", () => {
+    const r = validateIdeaSubmission(rawIdea({ targetRepo: { kind: "existing", repo: "not-a-slug" } }));
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.errors).toEqual(["target_repo_malformed"]);
+  });
+
+  it("flags an { kind: 'existing' } target whose repo is missing, empty, or not a string as target_repo_malformed (#9609)", () => {
+    for (const targetRepo of [{ kind: "existing" }, { kind: "existing", repo: "" }, { kind: "existing", repo: "   " }, { kind: "existing", repo: 42 }]) {
+      const r = validateIdeaSubmission(rawIdea({ targetRepo }));
+      expect(r.ok).toBe(false);
+      if (!r.ok) expect(r.errors).toEqual(["target_repo_malformed"]);
     }
   });