diff --git a/apps/server/src/provider/Layers/ClaudeAdapter.test.ts b/apps/server/src/provider/Layers/ClaudeAdapter.test.ts
index 31c56646c4f..672645c3d5b 100644
--- a/apps/server/src/provider/Layers/ClaudeAdapter.test.ts
+++ b/apps/server/src/provider/Layers/ClaudeAdapter.test.ts
@@ -37,6 +37,7 @@ import { ServerConfig } from "../../config.ts";
 import { ServerSettingsService } from "../../serverSettings.ts";
 import { ProviderAdapterValidationError } from "../Errors.ts";
 import type { ClaudeAdapterShape } from "../Services/ClaudeAdapter.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 import { makeClaudeAdapter, type ClaudeAdapterLiveOptions } from "./ClaudeAdapter.ts";
 import type { ClaudeRtkRewriteRunner } from "./ClaudeRtkToolRewrite.ts";
 const decodeClaudeSettings = Schema.decodeSync(ClaudeSettings);
@@ -411,6 +412,24 @@ describe("ClaudeAdapterLive", () => {
     );
   });
 
+  it.effect("injects GITS_PORT into Claude SDK query env", () => {
+    const harness = makeHarness();
+    return Effect.gen(function* () {
+      const adapter = yield* ClaudeAdapter;
+      yield* adapter.startSession({
+        threadId: THREAD_ID,
+        provider: ProviderDriverKind.make("claudeAgent"),
+        runtimeMode: "full-access",
+      });
+
+      const createInput = harness.getLastCreateQueryInput();
+      assert.equal(createInput?.options.env?.GITS_PORT, sessionPortEnv(THREAD_ID).GITS_PORT);
+    }).pipe(
+      Effect.provideService(Random.Random, makeDeterministicRandomService()),
+      Effect.provide(harness.layer),
+    );
+  });
+
   it.effect("maps the Claude Opus 4.7 default effort to the SDK-supported max value", () => {
     const harness = makeHarness();
     return Effect.gen(function* () {
diff --git a/apps/server/src/provider/Layers/ClaudeAdapter.ts b/apps/server/src/provider/Layers/ClaudeAdapter.ts
index bd92fc404ff..74bb11b833c 100644
--- a/apps/server/src/provider/Layers/ClaudeAdapter.ts
+++ b/apps/server/src/provider/Layers/ClaudeAdapter.ts
@@ -94,6 +94,7 @@ import {
 import { type ClaudeAdapterShape } from "../Services/ClaudeAdapter.ts";
 import { type EventNdjsonLogger, makeEventNdjsonLogger } from "./EventNdjsonLogger.ts";
 import { type GitShimManagerShape } from "../GitShimManager.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 const encodeUnknownJsonStringExit = Schema.encodeUnknownExit(Schema.UnknownFromJsonString);
 const decodeUnknownJsonStringExit = Schema.decodeUnknownExit(Schema.UnknownFromJsonString);
 
@@ -1055,11 +1056,11 @@ export const makeClaudeAdapter = Effect.fn("makeClaudeAdapter")(function* (
         options: input.options,
       }) as ClaudeQueryRuntime);
 
-  // Git shim manager injected at construction; per-session shim env stored here.
-  // CONFINEMENT LINE: sessionShimEnvs are merged into queryOptions.env only —
+  // Git shim manager injected at construction; per-session child env stored here.
+  // CONFINEMENT LINE: sessionChildEnvs are merged into queryOptions.env only —
   // claudeEnvironment (instance-level) and server process.env are never mutated.
   const gitShimManager: GitShimManagerShape | undefined = options?.gitShimManager;
-  const sessionShimEnvs = new Map<ThreadId, Record<string, string>>();
+  const sessionChildEnvs = new Map<ThreadId, Record<string, string>>();
 
   const sessions = new Map<ThreadId, ClaudeSessionContext>();
   const runtimeEventQueue = yield* Queue.bounded<ProviderRuntimeEvent>(
@@ -2571,7 +2572,7 @@ export const makeClaudeAdapter = Effect.fn("makeClaudeAdapter")(function* (
     }
 
     sessions.delete(context.session.threadId);
-    sessionShimEnvs.delete(context.session.threadId);
+    sessionChildEnvs.delete(context.session.threadId);
     // Clean up the per-session shim dir when the session stops.
     if (gitShimManager) yield* gitShimManager.release(context.session.threadId);
   });
@@ -2999,6 +3000,27 @@ export const makeClaudeAdapter = Effect.fn("makeClaudeAdapter")(function* (
         ...(fastMode ? { fastMode: true } : {}),
         ...(ultracode ? { ultracode: true } : {}),
       };
+      // Allocate per-session child env before creating the Claude SDK query;
+      // createQuery is the child-process spawn boundary for Claude.
+      // CONFINEMENT LINE: child vars merge into a claudeEnvironment copy only.
+      // Cleanup happens in stopSessionInternal via gitShimManager.release().
+      // ponytail: shim denied/warn events log via Codex adapter's stderr pipeline;
+      //   Claude SDK stderr is not captured here — add stderr capture when needed.
+      const childEnv = sessionPortEnv(threadId);
+      const sessionChildEnv = gitShimManager
+        ? {
+            ...childEnv,
+            ...(yield* gitShimManager
+              .allocate(threadId, input.cwd ?? "")
+              .pipe(
+                Effect.mapError((cause) =>
+                  toProcessError(cause, "Failed to allocate git confinement shim.", threadId),
+                ),
+              )).vars,
+          }
+        : childEnv;
+      sessionChildEnvs.set(threadId, sessionChildEnv);
+
       const queryOptions: ClaudeQueryOptions = {
         ...(input.cwd ? { cwd: input.cwd } : {}),
         ...(apiModelId ? { model: apiModelId } : {}),
@@ -3026,10 +3048,10 @@ export const makeClaudeAdapter = Effect.fn("makeClaudeAdapter")(function* (
         includePartialMessages: true,
         canUseTool,
         ...(visualPlanMcpServers ? { mcpServers: visualPlanMcpServers } : {}),
-        // Merge per-session shim env vars (GITS_ALLOWED_ROOT, PATH prepend, etc.)
-        // into the claude env. claudeEnvironment is the instance-level base; shimVars
+        // Merge per-session env vars (GITS_PORT, GITS_ALLOWED_ROOT, PATH prepend, etc.)
+        // into the claude env. claudeEnvironment is the instance-level base; child vars
         // add the per-session policy. Neither object is mutated.
-        env: Object.assign({}, claudeEnvironment, sessionShimEnvs.get(threadId) ?? {}),
+        env: Object.assign({}, claudeEnvironment, sessionChildEnv),
         ...(input.cwd ? { additionalDirectories: [input.cwd] } : {}),
         ...(Object.keys(extraArgs).length > 0 ? { extraArgs } : {}),
       };
@@ -3078,7 +3100,14 @@ export const makeClaudeAdapter = Effect.fn("makeClaudeAdapter")(function* (
             detail: toMessage(cause, "Failed to start Claude runtime session."),
             cause,
           }),
-      });
+      }).pipe(
+        Effect.tapError(() =>
+          Effect.gen(function* () {
+            sessionChildEnvs.delete(threadId);
+            if (gitShimManager) yield* gitShimManager.release(threadId);
+          }),
+        ),
+      );
 
       const session: ProviderSession = {
         threadId,
@@ -3123,23 +3152,6 @@ export const makeClaudeAdapter = Effect.fn("makeClaudeAdapter")(function* (
       yield* Ref.set(contextRef, context);
       sessions.set(threadId, context);
 
-      // Allocate git confinement shim for this session (non-supervisor sessions only).
-      // CONFINEMENT LINE: shimVars merged into claudeEnvironment copy per sendTurn only —
-      // claudeEnvironment (instance-level) and server process.env are never mutated.
-      // Cleanup happens in stopSessionInternal via gitShimManager.release().
-      // ponytail: shim denied/warn events log via Codex adapter's stderr pipeline;
-      //   Claude SDK stderr is not captured here — add stderr capture when needed.
-      if (gitShimManager) {
-        const shimResult = yield* gitShimManager
-          .allocate(threadId, input.cwd ?? "")
-          .pipe(
-            Effect.mapError((cause) =>
-              toProcessError(cause, "Failed to allocate git confinement shim.", threadId),
-            ),
-          );
-        sessionShimEnvs.set(threadId, shimResult.vars);
-      }
-
       const sessionStartedStamp = yield* makeEventStamp();
       yield* offerRuntimeEvent({
         type: "session.started",
diff --git a/apps/server/src/provider/Layers/CodexAdapter.test.ts b/apps/server/src/provider/Layers/CodexAdapter.test.ts
index 4e228e2a130..89c759e87d9 100644
--- a/apps/server/src/provider/Layers/CodexAdapter.test.ts
+++ b/apps/server/src/provider/Layers/CodexAdapter.test.ts
@@ -39,6 +39,7 @@ import { ServerSettingsService } from "../../serverSettings.ts";
 import { ProviderAdapterValidationError } from "../Errors.ts";
 import type { CodexAdapterShape } from "../Services/CodexAdapter.ts";
 import { ProviderSessionDirectory } from "../Services/ProviderSessionDirectory.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 import {
   type CodexSessionRuntimeOptions,
   type CodexForkResumeCursor,
@@ -292,6 +293,7 @@ validationLayer("CodexAdapterLive validation", (it) => {
       assert.deepStrictEqual(validationRuntimeFactory.factory.mock.calls[0]?.[0], {
         binaryPath: "codex",
         cwd: process.cwd(),
+        environment: sessionPortEnv("thread-1"),
         model: "gpt-5.3-codex",
         providerInstanceId: ProviderInstanceId.make("codex"),
         serviceTier: "fast",
@@ -407,6 +409,23 @@ sessionErrorLayer("CodexAdapterLive session errors", (it) => {
     }),
   );
 
+  it.effect("injects GITS_PORT into runtime environment", () =>
+    Effect.gen(function* () {
+      const adapter = yield* CodexAdapter;
+      const threadId = asThreadId("sess-gits-port-codex");
+
+      yield* adapter.startSession({
+        provider: ProviderDriverKind.make("codex"),
+        threadId,
+        runtimeMode: "full-access",
+      });
+
+      const runtime = sessionRuntimeFactory.lastRuntime;
+      assert.ok(runtime);
+      assert.equal(runtime.options.environment?.GITS_PORT, sessionPortEnv(threadId).GITS_PORT);
+    }),
+  );
+
   it.effect("maps codex model options for the adapter's bound custom instance id", () => {
     const customInstanceId = ProviderInstanceId.make("codex_personal");
     const customRuntimeFactory = makeRuntimeFactory();
diff --git a/apps/server/src/provider/Layers/CodexAdapter.ts b/apps/server/src/provider/Layers/CodexAdapter.ts
index 07346eb9035..735741836f0 100644
--- a/apps/server/src/provider/Layers/CodexAdapter.ts
+++ b/apps/server/src/provider/Layers/CodexAdapter.ts
@@ -67,6 +67,7 @@ import {
 } from "./CodexSessionRuntime.ts";
 import { type EventNdjsonLogger, makeEventNdjsonLogger } from "./EventNdjsonLogger.ts";
 import { type GitShimManagerShape } from "../GitShimManager.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 const isCodexAppServerProcessExitedError = Schema.is(CodexErrors.CodexAppServerProcessExitedError);
 const isCodexAppServerTransportError = Schema.is(CodexErrors.CodexAppServerTransportError);
 const isCodexSessionRuntimeThreadIdMissingError = Schema.is(
@@ -1440,10 +1441,12 @@ export const makeCodexAdapter = Effect.fn("makeCodexAdapter")(function* (
         if (options?.gitShimManager && Object.keys(shimEnv).length > 0) {
           yield* Scope.addFinalizer(sessionScope, options.gitShimManager.release(input.threadId));
         }
-        const sessionEnv: NodeJS.ProcessEnv | undefined =
-          options?.environment != null || Object.keys(shimEnv).length > 0
-            ? { ...(options?.environment ?? {}), ...shimEnv }
-            : undefined;
+        const sessionEnv: NodeJS.ProcessEnv = Object.assign(
+          {},
+          options?.environment,
+          sessionPortEnv(input.threadId),
+          shimEnv,
+        );
 
         const runtimeInput: CodexSessionRuntimeOptions = {
           threadId: input.threadId,
@@ -1451,7 +1454,7 @@ export const makeCodexAdapter = Effect.fn("makeCodexAdapter")(function* (
           cwd: sessionCwd,
           binaryPath: codexConfig.binaryPath,
           visualPlanMcpUrl: `http://127.0.0.1:${serverConfig.port}${VISUAL_PLAN_MCP_PATH}`,
-          ...(sessionEnv != null ? { environment: sessionEnv } : {}),
+          environment: sessionEnv,
           ...(codexConfig.homePath ? { homePath: codexConfig.homePath } : {}),
           ...(isCodexResumeCursorSchema(input.resumeCursor)
             ? { resumeCursor: input.resumeCursor }
diff --git a/apps/server/src/provider/Layers/CursorAdapter.test.ts b/apps/server/src/provider/Layers/CursorAdapter.test.ts
index 0c54c92b6dd..73256f51890 100644
--- a/apps/server/src/provider/Layers/CursorAdapter.test.ts
+++ b/apps/server/src/provider/Layers/CursorAdapter.test.ts
@@ -27,6 +27,7 @@ import {
 import { ServerConfig } from "../../config.ts";
 import { ServerSettingsService } from "../../serverSettings.ts";
 import type { CursorAdapterShape } from "../Services/CursorAdapter.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 import { makeCursorAdapter } from "./CursorAdapter.ts";
 const decodeCursorSettings = Schema.decodeSync(CursorSettings);
 
@@ -80,6 +81,18 @@ exec ${JSON.stringify(bunExe)} ${JSON.stringify(mockAgentPath)} "$@"
   return wrapperPath;
 }
 
+async function makeEnvProbeWrapper(envLogPath: string) {
+  const dir = await mkdtemp(path.join(os.tmpdir(), "cursor-acp-env-probe-"));
+  const wrapperPath = path.join(dir, "fake-agent.sh");
+  const script = `#!/bin/sh
+printf '%s\n' "\${GITS_PORT:-}" > ${JSON.stringify(envLogPath)}
+exec ${JSON.stringify(bunExe)} ${JSON.stringify(mockAgentPath)} "$@"
+`;
+  await writeFile(wrapperPath, script, "utf8");
+  await chmod(wrapperPath, 0o755);
+  return wrapperPath;
+}
+
 async function readArgvLog(filePath: string) {
   const raw = await readFile(filePath, "utf8");
   return raw
@@ -231,6 +244,32 @@ cursorAdapterTestLayer("CursorAdapterLive", (it) => {
     }),
   );
 
+  it.effect("injects GITS_PORT into spawned ACP process env", () =>
+    Effect.gen(function* () {
+      const adapter = yield* CursorAdapter;
+      const settings = yield* ServerSettingsService;
+      const threadId = ThreadId.make("cursor-gits-port-thread");
+      const tempDir = yield* Effect.promise(() =>
+        mkdtemp(path.join(os.tmpdir(), "cursor-gits-port-")),
+      );
+      const envLogPath = path.join(tempDir, "env.log");
+
+      const wrapperPath = yield* Effect.promise(() => makeEnvProbeWrapper(envLogPath));
+      yield* settings.updateSettings({ providers: { cursor: { binaryPath: wrapperPath } } });
+
+      yield* adapter.startSession({
+        threadId,
+        provider: ProviderDriverKind.make("cursor"),
+        cwd: process.cwd(),
+        runtimeMode: "full-access",
+      });
+
+      const raw = yield* Effect.promise(() => waitForFileContent(envLogPath));
+      assert.equal(raw.trim(), sessionPortEnv(threadId).GITS_PORT);
+      yield* adapter.stopSession(threadId);
+    }),
+  );
+
   it.effect("closes the ACP child process when a session stops", () =>
     Effect.gen(function* () {
       const adapter = yield* CursorAdapter;
diff --git a/apps/server/src/provider/Layers/CursorAdapter.ts b/apps/server/src/provider/Layers/CursorAdapter.ts
index ce57adebf77..05c81db688d 100644
--- a/apps/server/src/provider/Layers/CursorAdapter.ts
+++ b/apps/server/src/provider/Layers/CursorAdapter.ts
@@ -81,6 +81,7 @@ import { type CursorAdapterShape } from "../Services/CursorAdapter.ts";
 import { resolveCursorAcpBaseModelId } from "./CursorProvider.ts";
 import { type EventNdjsonLogger, makeEventNdjsonLogger } from "./EventNdjsonLogger.ts";
 import { type GitShimManagerShape } from "../GitShimManager.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 const encodeUnknownJsonStringExit = Schema.encodeUnknownExit(Schema.UnknownFromJsonString);
 
 const PROVIDER = ProviderDriverKind.make("cursor");
@@ -559,14 +560,16 @@ export function makeCursorAdapter(
           if (options?.gitShimManager && Object.keys(cursorShimEnv).length > 0) {
             yield* Scope.addFinalizer(sessionScope, options.gitShimManager.release(input.threadId));
           }
-          const cursorSessionEnv: NodeJS.ProcessEnv | undefined =
-            options?.environment != null || Object.keys(cursorShimEnv).length > 0
-              ? { ...(options?.environment ?? {}), ...cursorShimEnv }
-              : undefined;
+          const cursorSessionEnv: NodeJS.ProcessEnv = Object.assign(
+            {},
+            options?.environment,
+            sessionPortEnv(input.threadId),
+            cursorShimEnv,
+          );
 
           const acp = yield* makeCursorAcpRuntime({
             cursorSettings: effectiveCursorSettings,
-            ...(cursorSessionEnv != null ? { environment: cursorSessionEnv } : {}),
+            environment: cursorSessionEnv,
             childProcessSpawner,
             cwd,
             ...(resumeSessionId ? { resumeSessionId } : {}),
diff --git a/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts b/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
index 4849272035c..fec819d14b6 100644
--- a/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
+++ b/apps/server/src/provider/Layers/OpenCodeAdapter.test.ts
@@ -24,6 +24,7 @@ import { ServerConfig } from "../../config.ts";
 import { ServerSettingsService } from "../../serverSettings.ts";
 import { ProviderSessionDirectory } from "../Services/ProviderSessionDirectory.ts";
 import type { OpenCodeAdapterShape } from "../Services/OpenCodeAdapter.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 import {
   OpenCodeRuntime,
   OpenCodeRuntimeError,
@@ -55,6 +56,7 @@ const runtimeMock = {
     startCalls: [] as string[],
     sessionCreateUrls: [] as string[],
     authHeaders: [] as Array<string | null>,
+    connectEnvironments: [] as Array<NodeJS.ProcessEnv | undefined>,
     abortCalls: [] as string[],
     closeCalls: [] as string[],
     revertCalls: [] as Array<{ sessionID: string; messageID?: string }>,
@@ -68,6 +70,7 @@ const runtimeMock = {
     this.state.startCalls.length = 0;
     this.state.sessionCreateUrls.length = 0;
     this.state.authHeaders.length = 0;
+    this.state.connectEnvironments.length = 0;
     this.state.abortCalls.length = 0;
     this.state.closeCalls.length = 0;
     this.state.revertCalls.length = 0;
@@ -97,8 +100,9 @@ const OpenCodeRuntimeTestDouble: OpenCodeRuntimeShape = {
         exitCode: Effect.never,
       };
     }),
-  connectToOpenCodeServer: ({ serverUrl }) =>
+  connectToOpenCodeServer: ({ serverUrl, environment }) =>
     Effect.gen(function* () {
+      runtimeMock.state.connectEnvironments.push(environment);
       const url = serverUrl ?? "http://127.0.0.1:4301";
       // Unconditionally register a scope finalizer for test observability —
       // preserves the `closeCalls` / `closeError` probes that the existing
@@ -245,6 +249,10 @@ it.layer(OpenCodeAdapterTestLayer)("OpenCodeAdapterLive", (it) => {
       assert.deepEqual(runtimeMock.state.authHeaders, [
         `Basic ${btoa("opencode:secret-password")}`,
       ]);
+      assert.equal(
+        runtimeMock.state.connectEnvironments[0]?.GITS_PORT,
+        sessionPortEnv("thread-opencode").GITS_PORT,
+      );
     }),
   );
 
diff --git a/apps/server/src/provider/Layers/OpenCodeAdapter.ts b/apps/server/src/provider/Layers/OpenCodeAdapter.ts
index 84cd9b0c57e..024a5e58d66 100644
--- a/apps/server/src/provider/Layers/OpenCodeAdapter.ts
+++ b/apps/server/src/provider/Layers/OpenCodeAdapter.ts
@@ -28,6 +28,7 @@ import { resolveAttachmentPath } from "../../attachmentStore.ts";
 import { ServerConfig } from "../../config.ts";
 import { type EventNdjsonLogger, makeEventNdjsonLogger } from "./EventNdjsonLogger.ts";
 import { type GitShimManagerShape } from "../GitShimManager.ts";
+import { sessionPortEnv } from "../sessionPort.ts";
 import {
   ProviderAdapterProcessError,
   ProviderAdapterRequestError,
@@ -1052,10 +1053,12 @@ export function makeOpenCodeAdapter(
               .allocate(input.threadId, directory)
               .pipe(Effect.mapError((cause) => toProcessError(input.threadId, cause)))).vars
           : {};
-        const openCodeSessionEnv: NodeJS.ProcessEnv | undefined =
-          options?.environment != null || Object.keys(openCodeShimEnv).length > 0
-            ? { ...(options?.environment ?? {}), ...openCodeShimEnv }
-            : undefined;
+        const openCodeSessionEnv: NodeJS.ProcessEnv = Object.assign(
+          {},
+          options?.environment,
+          sessionPortEnv(input.threadId),
+          openCodeShimEnv,
+        );
 
         const started = yield* Effect.gen(function* () {
           const sessionScope = yield* Scope.make();
@@ -1070,7 +1073,7 @@ export function makeOpenCodeAdapter(
               const server = yield* openCodeRuntime.connectToOpenCodeServer({
                 binaryPath,
                 serverUrl,
-                ...(openCodeSessionEnv != null ? { environment: openCodeSessionEnv } : {}),
+                environment: openCodeSessionEnv,
               });
               const client = openCodeRuntime.createOpenCodeSdkClient({
                 baseUrl: server.url,
diff --git a/apps/server/src/provider/sessionPort.test.ts b/apps/server/src/provider/sessionPort.test.ts
new file mode 100644
index 00000000000..75f5b4e83f0
--- /dev/null
+++ b/apps/server/src/provider/sessionPort.test.ts
@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+
+import { sessionPortEnv, sessionPortForSessionId } from "./sessionPort.ts";
+
+describe("sessionPortForSessionId", () => {
+  it("returns a stable high port for the same session id", () => {
+    const first = sessionPortForSessionId("thread-session-a");
+    const second = sessionPortForSessionId("thread-session-a");
+
+    expect(first).toBe(second);
+    expect(first).toBeGreaterThanOrEqual(30_000);
+    expect(first).toBeLessThan(40_000);
+  });
+
+  it("spreads typical session ids across different ports", () => {
+    const ports = Array.from({ length: 128 }, (_, index) =>
+      sessionPortForSessionId(`thread-${index}`),
+    );
+
+    expect(new Set(ports).size).toBe(ports.length);
+  });
+
+  it("serializes the port as GITS_PORT env", () => {
+    expect(sessionPortEnv("thread-session-a")).toEqual({
+      GITS_PORT: String(sessionPortForSessionId("thread-session-a")),
+    });
+  });
+});
diff --git a/apps/server/src/provider/sessionPort.ts b/apps/server/src/provider/sessionPort.ts
new file mode 100644
index 00000000000..ef49c65d0f2
--- /dev/null
+++ b/apps/server/src/provider/sessionPort.ts
@@ -0,0 +1,17 @@
+const SESSION_PORT_BASE = 30_000;
+const SESSION_PORT_SPAN = 10_000;
+
+export function sessionPortForSessionId(sessionId: string): number {
+  let hash = 0x811c9dc5;
+  for (let index = 0; index < sessionId.length; index += 1) {
+    hash ^= sessionId.charCodeAt(index);
+    hash = Math.imul(hash, 0x01000193);
+  }
+  // ponytail: deterministic hash only; replace with a real allocator if active
+  // sessions ever collide in the 30000-39999 dev-server range.
+  return SESSION_PORT_BASE + ((hash >>> 0) % SESSION_PORT_SPAN);
+}
+
+export function sessionPortEnv(sessionId: string): Record<string, string> {
+  return { GITS_PORT: String(sessionPortForSessionId(sessionId)) };
+}
diff --git a/apps/server/src/terminal/Layers/Manager.test.ts b/apps/server/src/terminal/Layers/Manager.test.ts
index c7a321d1195..4b3ae5bb0f4 100644
--- a/apps/server/src/terminal/Layers/Manager.test.ts
+++ b/apps/server/src/terminal/Layers/Manager.test.ts
@@ -26,6 +26,7 @@ import { TestClock } from "effect/testing";
 import { expect } from "vitest";
 
 import * as ProcessRunner from "../../processRunner.ts";
+import { sessionPortEnv } from "../../provider/sessionPort.ts";
 import type { TerminalManagerShape } from "../Services/Manager.ts";
 import {
   type PtyAdapterShape,
@@ -1229,6 +1230,23 @@ it.layer(
     }),
   );
 
+  it.effect("injects deterministic GITS_PORT into spawned terminals", () =>
+    Effect.gen(function* () {
+      const { manager, ptyAdapter } = yield* createManager();
+      yield* manager.open(
+        openInput({
+          threadId: "thread-terminal-port",
+          env: { GITS_PORT: "9999" },
+        }),
+      );
+      const spawnInput = ptyAdapter.spawnInputs[0];
+      expect(spawnInput).toBeDefined();
+      if (!spawnInput) return;
+
+      assert.equal(spawnInput.env.GITS_PORT, sessionPortEnv("thread-terminal-port").GITS_PORT);
+    }),
+  );
+
   it.effect("starts zsh with prompt spacer disabled to avoid `%` end markers", () =>
     Effect.gen(function* () {
       if (process.platform === "win32") return;
diff --git a/apps/server/src/terminal/Layers/Manager.ts b/apps/server/src/terminal/Layers/Manager.ts
index 82eb9502cf6..53df280ef00 100644
--- a/apps/server/src/terminal/Layers/Manager.ts
+++ b/apps/server/src/terminal/Layers/Manager.ts
@@ -48,6 +48,7 @@ import {
   type PtyExitEvent,
   type PtyProcess,
 } from "../Services/PTY.ts";
+import { sessionPortEnv } from "../../provider/sessionPort.ts";
 
 const DEFAULT_HISTORY_LINE_LIMIT = 5_000;
 const DEFAULT_PERSIST_DEBOUNCE_MS = 40;
@@ -1632,7 +1633,10 @@ export const makeTerminalManagerWithOptions = Effect.fn("makeTerminalManagerWith
           Effect.andThen(
             Effect.gen(function* () {
               const shellCandidates = resolveShellCandidates(shellResolver, platform, baseEnv);
-              const terminalEnv = createTerminalSpawnEnv(baseEnv, session.runtimeEnv);
+              const portEnv = sessionPortEnv(session.threadId);
+              const terminalRuntimeEnv =
+                session.runtimeEnv === null ? portEnv : { ...session.runtimeEnv, ...portEnv };
+              const terminalEnv = createTerminalSpawnEnv(baseEnv, terminalRuntimeEnv);
               const spawnResult = yield* trySpawn(shellCandidates, terminalEnv, session);
               ptyProcess = spawnResult.process;
               startedShell = spawnResult.shellLabel;