diff --git a/src/providers/key-store.ts b/src/providers/key-store.ts
index 12e4ce6cb7..614fd3372f 100644
--- a/src/providers/key-store.ts
+++ b/src/providers/key-store.ts
@@ -64,6 +64,16 @@ function keychainAccount(reference: string): string {
   return reference.slice(KEYCHAIN_REFERENCE_PREFIX.length);
 }
 
+/**
+ * A reference belongs to `name` only when its account is that provider's own active account
+ * or one of its pool accounts. `storeProviderKeyInKeychain` writes exactly those two shapes,
+ * so anything else in a provider's config names another provider's secret.
+ */
+function keychainReferenceBelongsToProvider(reference: string, name: string): boolean {
+  const account = keychainAccount(reference);
+  return account === name || account.startsWith(`${name}/`);
+}
+
 function readKeychain(account: string): string | undefined {
   const cached = resolvedCache.get(account);
   if (cached !== undefined) return cached;
@@ -185,6 +195,18 @@ export function restoreProviderKeyFromKeychain(config: OcxConfig, name: string):
   const pool = provider.apiKeyPool ?? [];
   const resolved = new Map<string, string>();
   const refs = [provider.apiKey, ...pool.map(e => e.key)].filter(isKeychainReference);
+  // Restore reads a secret out of the keychain, writes it back to config as plaintext, and then
+  // DELETES the keychain item. Following a reference to another provider's account would both
+  // disclose that secret through this provider's config and destroy the real owner's credential,
+  // so refuse before anything is read or removed.
+  const foreign = refs.filter(ref => !keychainReferenceBelongsToProvider(ref, name));
+  if (foreign.length > 0) {
+    return {
+      ok: false,
+      error: `provider "${name}" references a keychain account it does not own (${foreign.length} reference(s)); config left unchanged`,
+      status: 400,
+    };
+  }
   for (const ref of refs) {
     const account = keychainAccount(ref);
     if (resolved.has(account)) continue;
diff --git a/src/responses/citation-markers.ts b/src/responses/citation-markers.ts
index 5fe58142cf..3e3943cad5 100644
--- a/src/responses/citation-markers.ts
+++ b/src/responses/citation-markers.ts
@@ -68,6 +68,15 @@ export interface CitationMarkerFilter {
   flush(): string;
 }
 
+/**
+ * Upper bound on the text withheld for one unterminated START.
+ *
+ * A real span is `cite` plus a few turn-scoped ids, so it is far under this. Without a
+ * bound, a backend that emits a START and never terminates it makes `held` grow for the
+ * whole response, and every later delta re-scans that accumulated prefix.
+ */
+const MAX_STREAMING_MARKER_SPAN_LENGTH = 4_096;
+
 /**
  * Streaming filter.
  *
@@ -75,6 +84,9 @@ export interface CitationMarkerFilter {
  * next — so a stateless per-delta strip would emit the tail of a span it never recognized.
  * This holds back the text from an unterminated START and releases it once the END arrives
  * (removed) or the stream ends (verbatim, so nothing the model actually said is lost).
+ *
+ * A span that grows past `MAX_STREAMING_MARKER_SPAN_LENGTH` is malformed ordinary text, so
+ * it is released verbatim instead of withheld; a later START can still open a valid span.
  */
 export function createCitationMarkerFilter(): CitationMarkerFilter {
   // Text from an open START that has not been terminated yet.
@@ -83,13 +95,29 @@ export function createCitationMarkerFilter(): CitationMarkerFilter {
     push(delta: string): string {
       const combined = held + delta;
       held = "";
-      const start = combined.lastIndexOf(CITATION_MARKER_START);
-      if (start === -1) return stripCitationMarkers(combined);
-      const endAfterStart = combined.indexOf(CITATION_MARKER_END, start + 1);
-      if (endAfterStart !== -1) return stripCitationMarkers(combined);
-      // The trailing span is still open: emit everything before it, hold the rest.
-      held = combined.slice(start);
-      return stripCitationMarkers(combined.slice(0, start));
+      let start = combined.indexOf(CITATION_MARKER_START);
+      if (start === -1) return combined;
+      let out = combined.slice(0, start);
+      // Walk START-delimited segments independently so an earlier malformed START is never
+      // paired with a later span's END (the whole-string strip would do exactly that).
+      while (start !== -1) {
+        const nextStart = combined.indexOf(CITATION_MARKER_START, start + 1);
+        const segment = combined.slice(start, nextStart === -1 ? combined.length : nextStart);
+        const end = segment.indexOf(CITATION_MARKER_END, 1);
+        if (end !== -1) {
+          // A complete span: drop it, keep whatever trails it inside this segment.
+          out += segment.slice(end + 1);
+        } else if (nextStart === -1 && segment.length <= MAX_STREAMING_MARKER_SPAN_LENGTH) {
+          // Only a bounded trailing span can still be completed by a later delta.
+          held = segment;
+        } else {
+          // Superseded by a later START, or over the bound: ordinary text, emitted verbatim
+          // so neither the retained text nor the per-delta rescan grows without limit.
+          out += segment;
+        }
+        start = nextStart;
+      }
+      return out;
     },
     flush(): string {
       const rest = held;
@@ -98,4 +126,3 @@ export function createCitationMarkerFilter(): CitationMarkerFilter {
     },
   };
 }
-
diff --git a/tests/providers/provider-key-store.test.ts b/tests/providers/provider-key-store.test.ts
index 645920e32c..1197a1fea9 100644
--- a/tests/providers/provider-key-store.test.ts
+++ b/tests/providers/provider-key-store.test.ts
@@ -156,6 +156,37 @@ describe("store / restore", () => {
     expect(probeProviderKeychain().available).toBe(false);
   });
 
+  test("restore refuses a reference to another provider's keychain account", () => {
+    const { store, factory } = fakeKeychain();
+    setProviderKeychainEntryFactoryForTests(factory);
+    const config = loadConfig();
+    config.providers.other = { adapter: "openai-chat", baseUrl: "https://other.example/v1", apiKey: POOL_SECRET };
+    expect(storeProviderKeyInKeychain(config, "other")).toEqual({ ok: true, moved: 1 });
+    expect(config.providers.other!.apiKey).toBe("keychain:other");
+
+    // Point "relay" at the account "other" owns. Restore would otherwise read that secret,
+    // write it into relay's config as plaintext, and delete the owner's keychain item.
+    config.providers.relay!.apiKey = "keychain:other";
+    const result = restoreProviderKeyFromKeychain(config, "relay");
+    expect(result.ok).toBe(false);
+    if (!result.ok) expect(result.status).toBe(400);
+
+    expect(config.providers.relay!.apiKey).toBe("keychain:other");
+    expect(readFileSync(join(testDir, "config.json"), "utf8")).not.toContain(POOL_SECRET);
+    // The real owner's secret is still in the keychain and still resolves for that provider.
+    expect(store.size).toBe(1);
+    expect(resolveProviderApiKey(config.providers.other!.apiKey)).toBe(POOL_SECRET);
+  });
+
+  test("restore still accepts a provider's own active and pool accounts", () => {
+    const { factory } = fakeKeychain();
+    setProviderKeychainEntryFactoryForTests(factory);
+    const config = loadConfig();
+    config.providers.relay!.apiKeyPool = [{ id: "a1", key: SECRET }, { id: "b2", key: POOL_SECRET }];
+    expect(storeProviderKeyInKeychain(config, "relay")).toEqual({ ok: true, moved: 2 });
+    expect(restoreProviderKeyFromKeychain(config, "relay")).toEqual({ ok: true, restored: 2 });
+  });
+
   test("management route: GET reports store kind, POST store/restore round-trips", async () => {
     const { factory } = fakeKeychain();
     setProviderKeychainEntryFactoryForTests(factory);
@@ -192,4 +223,3 @@ describe("store / restore", () => {
     }
   });
 });
-
diff --git a/tests/responses/citation-markers.test.ts b/tests/responses/citation-markers.test.ts
index 0c1921750c..dcc7f9abc6 100644
--- a/tests/responses/citation-markers.test.ts
+++ b/tests/responses/citation-markers.test.ts
@@ -87,4 +87,34 @@ describe("streaming citation marker filter (#3150)", () => {
     const filter = createCitationMarkerFilter();
     expect(filter.push(`visible now ${S}cite`)).toBe("visible now ");
   });
+
+  test("an unterminated span past the bound is released instead of retained", () => {
+    // A backend that opens a span and never closes it must not make the filter accumulate
+    // the rest of the response, which every later delta would then re-scan.
+    const filter = createCitationMarkerFilter();
+    let out = filter.push(`kept ${S}cite`);
+    expect(out).toBe("kept ");
+    for (let i = 0; i < 5_000; i += 1) out += filter.push("x");
+
+    // Everything after the malformed START is emitted verbatim, so nothing is lost, and
+    // flush() has nothing left to release.
+    expect(out).toBe(`kept ${S}cite${"x".repeat(5_000)}`);
+    expect(filter.flush()).toBe("");
+  });
+
+  test("a later START still opens a valid span after a released malformed one", () => {
+    const filter = createCitationMarkerFilter();
+    let out = filter.push(`a${S}${"y".repeat(5_000)}`);
+    out += filter.push(`${S}cite${P}turn1view0${E} tail`);
+    expect(out).toBe(`a${S}${"y".repeat(5_000)} tail`);
+    expect(filter.flush()).toBe("");
+  });
+
+  test("an oversized malformed span survives a later valid marker in the same delta", () => {
+    const filter = createCitationMarkerFilter();
+    const malformed = `${S}${"y".repeat(5_000)}`;
+    expect(filter.push(`a${span}${malformed}${S}cite${P}turn1view0${E} tail`))
+      .toBe(`a${malformed} tail`);
+    expect(filter.flush()).toBe("");
+  });
 });