diff --git a/packages/client-runtime/src/state/boundedThreadSnapshotHttp.ts b/packages/client-runtime/src/state/boundedThreadSnapshotHttp.ts
index a049a6b14863..1a487e9b9d67 100644
--- a/packages/client-runtime/src/state/boundedThreadSnapshotHttp.ts
+++ b/packages/client-runtime/src/state/boundedThreadSnapshotHttp.ts
@@ -40,7 +40,10 @@ export const fetchEnvironmentBoundedThreadSnapshot = Effect.fn(
     group: "orchestration",
     method: "GET",
     url: (httpBaseUrl) =>
-      environmentEndpointUrl(httpBaseUrl, `/api/orchestration/threads/${input.threadId}/bounded`),
+      environmentEndpointUrl(
+        httpBaseUrl,
+        `/api/orchestration/threads/${encodeURIComponent(input.threadId)}/bounded`,
+      ),
     timeoutMs: input.timeoutMs ?? DEFAULT_BOUNDED_THREAD_SNAPSHOT_TIMEOUT_MS,
     request: ({ client, headers }) =>
       client.threadBoundedSnapshot({
diff --git a/packages/client-runtime/src/state/environmentHttpAuth.test.ts b/packages/client-runtime/src/state/environmentHttpAuth.test.ts
index 61504668427e..0a2df6f936a5 100644
--- a/packages/client-runtime/src/state/environmentHttpAuth.test.ts
+++ b/packages/client-runtime/src/state/environmentHttpAuth.test.ts
@@ -4,6 +4,7 @@ import {
   ORCHESTRATION_PROTOCOL_HEADER,
   ORCHESTRATION_PROTOCOL_VERSION_TEXT,
   ProjectId,
+  ThreadId,
   type AuthSessionState,
   type OrchestrationV2ShellSnapshot,
   OrchestrationV2ThreadDetailSnapshot,
@@ -307,6 +308,50 @@ describe("authenticated environment HTTP requests", () => {
     }),
   );
 
+  // MCP-created thread ids contain ":", which the request path percent-encodes.
+  // The DPoP proof must sign the URL that is actually sent, or the environment
+  // rejects it as a URL mismatch.
+  const MCP_THREAD_ID = ThreadId.make("mcp:3534bc83-1c17-4a1e-9118-601c2766d355");
+  const MCP_THREAD_LOADERS: ReadonlyArray<
+    Pick<(typeof LOADERS)[number], "name" | "response" | "load">
+  > = [
+    {
+      name: "thread snapshot",
+      response: encodeThreadSnapshot(THREAD),
+      load: (input: HttpInput) =>
+        ThreadSnapshotLoader.fetchEnvironmentThreadSnapshot({ ...input, threadId: MCP_THREAD_ID }),
+    },
+    {
+      name: "bounded thread snapshot",
+      response: encodeBoundedSnapshot(BOUNDED_THREAD),
+      load: (input: HttpInput) =>
+        fetchEnvironmentBoundedThreadSnapshot({ ...input, threadId: MCP_THREAD_ID }),
+    },
+    {
+      name: "older thread history",
+      response: THREAD_HISTORY,
+      load: (input: HttpInput) =>
+        fetchEnvironmentThreadHistoryPage({
+          ...input,
+          threadId: MCP_THREAD_ID,
+          cursor: "older-page",
+        }),
+    },
+  ];
+  it.effect.each(MCP_THREAD_LOADERS)(
+    "signs the sent URL for a $name of a thread id that needs encoding",
+    (loader) =>
+      Effect.gen(function* () {
+        const harness = makeHarness(() => Response.json(loader.response));
+        yield* loader.load(harness.input).pipe(Effect.provide(harness.httpLayer));
+
+        const sent = new URL(harness.calls[0]!.url);
+        expect(sent.pathname).toContain("/mcp%3A3534bc83-");
+        sent.search = "";
+        expect(harness.proofs.map((proof) => proof.url)).toEqual([sent.toString()]);
+      }),
+  );
+
   it.effect("retries a rejected diff once with a new token, endpoint, and proof", () =>
     Effect.gen(function* () {
       const harness = makeHarness((requestNumber) =>
diff --git a/packages/client-runtime/src/state/threadHistoryHttp.ts b/packages/client-runtime/src/state/threadHistoryHttp.ts
index b83cb6f20c35..316e0567305a 100644
--- a/packages/client-runtime/src/state/threadHistoryHttp.ts
+++ b/packages/client-runtime/src/state/threadHistoryHttp.ts
@@ -30,7 +30,10 @@ export const fetchEnvironmentThreadHistoryPage = Effect.fn(
     group: "orchestration",
     method: "GET",
     url: (httpBaseUrl) =>
-      environmentEndpointUrl(httpBaseUrl, `/api/orchestration/threads/${input.threadId}/history`),
+      environmentEndpointUrl(
+        httpBaseUrl,
+        `/api/orchestration/threads/${encodeURIComponent(input.threadId)}/history`,
+      ),
     timeoutMs: input.timeoutMs ?? DEFAULT_THREAD_HISTORY_TIMEOUT_MS,
     request: ({ client, headers }) =>
       client.threadHistoryPage({
diff --git a/packages/client-runtime/src/state/threadSnapshotHttp.ts b/packages/client-runtime/src/state/threadSnapshotHttp.ts
index 8b31e4c60687..905d8872d589 100644
--- a/packages/client-runtime/src/state/threadSnapshotHttp.ts
+++ b/packages/client-runtime/src/state/threadSnapshotHttp.ts
@@ -66,7 +66,10 @@ export const fetchEnvironmentThreadSnapshot = Effect.fn(
     group: "orchestration",
     method: "GET",
     url: (httpBaseUrl) =>
-      environmentEndpointUrl(httpBaseUrl, `/api/orchestration/threads/${input.threadId}`),
+      environmentEndpointUrl(
+        httpBaseUrl,
+        `/api/orchestration/threads/${encodeURIComponent(input.threadId)}`,
+      ),
     timeoutMs: input.timeoutMs ?? DEFAULT_THREAD_SNAPSHOT_TIMEOUT_MS,
     request: ({ client, headers }) =>
       client.threadSnapshot({