diff --git a/apps/server/src/git/GitManager.ts b/apps/server/src/git/GitManager.ts
index dfd13a7c06609..23a79020c10f6 100644
--- a/apps/server/src/git/GitManager.ts
+++ b/apps/server/src/git/GitManager.ts
@@ -18,6 +18,7 @@ import * as PubSub from "effect/PubSub";
 import * as Ref from "effect/Ref";
 import * as Schema from "effect/Schema";
 import * as Scope from "effect/Scope";
+import * as Semaphore from "effect/Semaphore";
 import * as Stream from "effect/Stream";
 import {
   GitActionProgressEvent,
@@ -189,6 +190,9 @@ const PR_LOOKUP_FAILURE_MAX_TTL = Duration.minutes(15);
 const PR_LOOKUP_CACHE_CAPACITY = 2_048;
 const PR_LOOKUP_GIT_STATE_TTL = Duration.minutes(5);
 const SETTLED_PR_LOOKUP_RECHECK_INTERVAL = Duration.minutes(15);
+// Shared by every background caller (backfill, settlement, cleanup), so startup discovery
+// cannot fill the git and forge process slots that foreground work also needs.
+const BRANCH_PR_LOOKUP_CONCURRENCY = 4;
 const PR_THREAD_CONTEXT_BYTES = 16_000;
 const PR_THREAD_COMMAND_OUTPUT_CHARS = 2_000;
 const isSourceControlProviderError = Schema.is(SourceControlProviderError);
@@ -2579,6 +2583,7 @@ export const make = Effect.gen(function* () {
     prLookupGitStateByKey.set(entryKey, { readAtMs, value });
     return value;
   });
+  const branchPrLookups = yield* Semaphore.make(BRANCH_PR_LOOKUP_CONCURRENCY);
   const settledPrLookupByBranchKey = new Map<
     string,
     { readonly url: string; readonly fingerprint: string; readonly confirmedAtMs: number }
@@ -2879,7 +2884,7 @@ export const make = Effect.gen(function* () {
       });
     }
     return confirmed;
-  });
+  }, branchPrLookups.withPermits(1));
 
   const checkedOutPullRequest: GitManager["Service"]["checkedOutPullRequest"] = Effect.fn(
     "checkedOutPullRequest",
diff --git a/apps/server/src/orchestration-v2/PullRequestSyncReactor.test.ts b/apps/server/src/orchestration-v2/PullRequestSyncReactor.test.ts
index 098c76184bbc9..ad134daab0f32 100644
--- a/apps/server/src/orchestration-v2/PullRequestSyncReactor.test.ts
+++ b/apps/server/src/orchestration-v2/PullRequestSyncReactor.test.ts
@@ -8,6 +8,7 @@ import {
   ProjectId,
   ProviderInstanceId,
   PullRequestOperationError,
+  PullRequestUnavailableError,
   RunId,
   ThreadId,
   TurnItemId,
@@ -180,7 +181,7 @@ interface HarnessOptions {
   readonly snapshot: TestShellSnapshot;
   readonly summary?: (
     input: PullRequestRef,
-  ) => Effect.Effect<PullRequestSummary, PullRequestOperationError>;
+  ) => Effect.Effect<PullRequestSummary, PullRequestService.PullRequestError>;
   readonly stack?: (
     input: PullRequestRef,
   ) => Effect.Effect<PullRequestStack | null, PullRequestOperationError>;
@@ -1674,6 +1675,45 @@ describe("PullRequestSyncReactor", () => {
       }),
   );
 
+  it.effect("stops reading a pull request on a host it cannot browse each sweep", () =>
+    Effect.gen(function* () {
+      yield* TestClock.setTime(Date.parse(NOW));
+      const fixture = yield* makeHarness({
+        snapshot: makeSnapshot([
+          makeThread("unsupported", { pullRequests: [makeLink(9, { state: "open" })] }),
+          makeThread("fine", { pullRequests: [makeLink(8, { state: "open" })] }),
+        ]),
+        summary: (input) =>
+          input.number === 9
+            ? Effect.fail(new PullRequestUnavailableError({ reason: "provider-unsupported" }))
+            : Effect.succeed(makeSummary(input)),
+      });
+      const unsupportedReads = Ref.get(fixture.summaryCalls).pipe(
+        Effect.map((calls) => calls.filter((call) => call.number === 9).length),
+      );
+      yield* Effect.gen(function* () {
+        const reactor = yield* startAndSweep(fixture);
+        for (let index = 0; index < 5; index += 1) yield* sweepAgain(fixture, reactor);
+        const afterSweeps = yield* unsupportedReads;
+
+        // A refresh another link asks for must not re-read the unsupported one as well.
+        yield* reactor.requestSync(makeLink(9));
+        yield* Queue.take(fixture.snapshotReads);
+        yield* reactor.drain;
+        const afterItsRefresh = yield* unsupportedReads;
+        for (let index = 0; index < 3; index += 1) {
+          yield* reactor.requestSync(makeLink(8));
+          yield* Queue.take(fixture.snapshotReads);
+          yield* reactor.drain;
+        }
+        assert.deepStrictEqual(
+          { afterSweeps, afterItsRefresh, afterOtherRefreshes: yield* unsupportedReads },
+          { afterSweeps: 1, afterItsRefresh: 2, afterOtherRefreshes: 2 },
+        );
+      }).pipe(Effect.provide(fixture.layer));
+    }),
+  );
+
   it.effect(
     "holds missing links through sibling and link updates but honors a reader refresh",
     () =>
diff --git a/apps/server/src/orchestration-v2/PullRequestSyncReactor.ts b/apps/server/src/orchestration-v2/PullRequestSyncReactor.ts
index d21b607f1bf58..46648e13b4b57 100644
--- a/apps/server/src/orchestration-v2/PullRequestSyncReactor.ts
+++ b/apps/server/src/orchestration-v2/PullRequestSyncReactor.ts
@@ -366,7 +366,8 @@ export const make = Effect.gen(function* () {
   // linking dozens of pull requests) is read together and shares the summary batches.
   let requestedSweepQueued = false;
   const retryStacks = new Set<string>();
-  // Group keys the host answered not-found for; they are read on the slow cadence until found.
+  // Group keys the host answered not-found for, or this server cannot read (no CLI, signed out,
+  // unsupported host); they are read on the slow cadence until a read succeeds.
   const missing = new Set<string>();
   // Pause per project and host because credentials are project-specific; leave paused PRs due until reset.
   const pausedUntil = new Map<string, number>();
@@ -510,9 +511,10 @@ export const make = Effect.gen(function* () {
       if (invalidated) yield* pullRequests.invalidate({ reference: ref });
       const summary = yield* pullRequests.summary(ref, { recoverTransientFailure: false }).pipe(
         Effect.tapError((error) =>
-          error.reason === "not-found"
+          error.reason === "not-found" || error._tag === "PullRequestUnavailableError"
             ? Effect.sync(() => {
-                // Not-found is a host answer: it consumes the refresh and starts the backoff.
+                // Not-found, or a host this server cannot read, won't change by asking again
+                // next sweep: it consumes the refresh and starts the backoff.
                 missing.add(key);
                 lastSyncedAt.set(key, nowMs);
                 requested.delete(key);