diff --git a/.github/scripts/check-source-stack.sh b/.github/scripts/check-source-stack.sh
new file mode 100644
index 00000000000..5597c5a920e
--- /dev/null
+++ b/.github/scripts/check-source-stack.sh
@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+# Verify that a manually supplied nightly source contains every fork patch
+# present on main. This catches accidentally stale stacks; it is not a
+# security boundary or a net-tree comparison (a later revert can still retain
+# an earlier commit's patch-id). The refs must already exist locally, and the
+# fork's squash-merged patch stack is expected to have linear history.
+set -euo pipefail
+
+main_ref="${1:?main ref is required}"
+candidate_ref="${2:?candidate ref is required}"
+upstream_ref="${3:?upstream ref is required}"
+allow_missing="${4:-false}"
+
+for ref in "$main_ref" "$candidate_ref" "$upstream_ref"; do
+  git rev-parse --verify "${ref}^{commit}" >/dev/null
+done
+
+cherry_output=$(git cherry "$candidate_ref" "$main_ref" "$upstream_ref")
+
+missing_commits=()
+while read -r status commit; do
+  if [[ "$status" == "+" ]]; then
+    missing_commits+=("$commit")
+  fi
+done <<< "$cherry_output"
+
+if (( ${#missing_commits[@]} == 0 )); then
+  echo "source_ref contains every patch currently carried by main."
+  exit 0
+fi
+
+{
+  echo "source_ref is missing patch-id-equivalent commits currently carried by main:"
+  echo
+  for commit in "${missing_commits[@]}"; do
+    printf '  %s %s\n' \
+      "$(git rev-parse --short=12 "$commit")" \
+      "$(git show -s --format=%s "$commit")"
+  done
+  echo
+} >&2
+
+if [[ "$allow_missing" == "true" ]]; then
+  echo "Conflict resolution may intentionally reshape patches; continuing after the explicit override." >&2
+  echo "::warning title=Missing main patches explicitly allowed::Continuing because allow_missing_main_patches was enabled." >&2
+  exit 0
+fi
+
+echo "Refresh the resolution from current main before dispatching it." >&2
+echo "If conflict resolution intentionally reshaped these patches, re-dispatch" >&2
+echo "with allow_missing_main_patches enabled after reviewing this list." >&2
+echo "::error title=source_ref is missing patches from main::Refresh the stack or explicitly allow the reported patch-id differences." >&2
+exit 1
diff --git a/.github/workflows/fork-nightly.yml b/.github/workflows/fork-nightly.yml
index 7988000ea64..820bde67c80 100644
--- a/.github/workflows/fork-nightly.yml
+++ b/.github/workflows/fork-nightly.yml
@@ -25,6 +25,13 @@ on:
         required: false
         type: string
         default: ""
+      allow_missing_main_patches:
+        description: >-
+          Allow source_ref when patch-id checks report main patches missing;
+          use only after reviewing intentionally reshaped conflict resolutions
+        required: false
+        type: boolean
+        default: false
 
 permissions:
   contents: write
@@ -93,6 +100,7 @@ jobs:
         name: Rebase candidate onto upstream
         shell: bash
         env:
+          ALLOW_MISSING_MAIN_PATCHES: ${{ inputs.allow_missing_main_patches == true }}
           DRY_RUN: ${{ inputs.dry_run == true }}
           SOURCE_REF: ${{ inputs.source_ref }}
         run: |
@@ -109,6 +117,23 @@ jobs:
               echo "origin has no main branch to promote onto." >&2
               exit 1
             fi
+
+            # Fetch the immutable main snapshot and current upstream before
+            # rebasing. The guard itself is loaded from main so a stale
+            # source_ref cannot omit or roll back the check it must pass.
+            git fetch --no-tags origin "$main_ref"
+            git fetch --no-tags https://github.com/pingdotgg/t3code.git main
+            guard_upstream_ref=$(git rev-parse FETCH_HEAD)
+            guard_script="$RUNNER_TEMP/check-source-stack.sh"
+            if ! git show "${main_ref}:.github/scripts/check-source-stack.sh" > "$guard_script"; then
+              echo "main snapshot ${main_ref} does not contain the source_ref patch guard; update main before dispatching a source_ref run." >&2
+              exit 1
+            fi
+            bash "$guard_script" \
+              "$main_ref" \
+              "$fork_ref" \
+              "$guard_upstream_ref" \
+              "$ALLOW_MISSING_MAIN_PATCHES"
           else
             main_ref=$fork_ref
           fi
diff --git a/docs/operations/fork-nightly.md b/docs/operations/fork-nightly.md
index 35ffd0d0ace..9152d40eebf 100644
--- a/docs/operations/fork-nightly.md
+++ b/docs/operations/fork-nightly.md
@@ -27,7 +27,12 @@ publishes a GitHub prerelease.
   when it did), verifies it, publishes the release, and promotes it to `main` through the same backup
   and lease mechanics as an automated run. The ruleset blocks force-pushing `main` from the CLI, so
   this dispatch is how a resolved stack reaches `main`. Pair `source_ref` with `dry_run` first to
-  verify a resolution without publishing or promoting anything.
+  verify a resolution without publishing or promoting anything. Prepare compares the supplied
+  stack's patch IDs with every fork patch currently carried by `main` and fails with the missing
+  commit subjects before rebasing or verifying a stale stack. Refresh the resolution from current
+  `main` immediately before dispatching it. Conflict resolution can intentionally reshape a patch
+  enough to change its patch ID; after reviewing every reported commit, set
+  `allow_missing_main_patches` to acknowledge those differences explicitly.
 
 ## Fork features summary
 
diff --git a/scripts/check-source-stack.test.ts b/scripts/check-source-stack.test.ts
new file mode 100644
index 00000000000..2bb69ee9eb5
--- /dev/null
+++ b/scripts/check-source-stack.test.ts
@@ -0,0 +1,221 @@
+// @effect-diagnostics nodeBuiltinImport:off
+import * as NodeChildProcess from "node:child_process";
+import * as NodeFS from "node:fs";
+import * as NodeOS from "node:os";
+import * as NodePath from "node:path";
+import * as NodeURL from "node:url";
+import { assert, describe, it } from "@effect/vitest";
+
+const repoRoot = NodePath.resolve(NodePath.dirname(NodeURL.fileURLToPath(import.meta.url)), "..");
+const helperPath = NodePath.resolve(repoRoot, ".github/scripts/check-source-stack.sh");
+
+function runGit(cwd: string, ...args: ReadonlyArray<string>): string {
+  const result = NodeChildProcess.spawnSync("git", args, { cwd, encoding: "utf8" });
+  if (result.error) throw result.error;
+  if (result.status !== 0) {
+    throw new Error(`git ${args.join(" ")} failed: ${result.stderr}`);
+  }
+  return result.stdout.trim();
+}
+
+function commitFile(cwd: string, fileName: string, contents: string, subject: string): string {
+  NodeFS.writeFileSync(NodePath.join(cwd, fileName), contents);
+  runGit(cwd, "add", fileName);
+  runGit(cwd, "commit", "-m", subject);
+  return runGit(cwd, "rev-parse", "HEAD");
+}
+
+function checkStack(
+  cwd: string,
+  mainRef: string,
+  candidateRef: string,
+  upstreamRef: string,
+  allowMissing = false,
+  env: NodeJS.ProcessEnv = process.env,
+): NodeChildProcess.SpawnSyncReturns<string> {
+  return NodeChildProcess.spawnSync(
+    "bash",
+    [helperPath, mainRef, candidateRef, upstreamRef, allowMissing ? "true" : "false"],
+    { cwd, encoding: "utf8", env },
+  );
+}
+
+function withRepository(run: (fixtureRoot: string, upstreamRef: string) => void): void {
+  const fixtureRoot = NodeFS.mkdtempSync(NodePath.join(NodeOS.tmpdir(), "t3-source-stack-"));
+
+  try {
+    runGit(fixtureRoot, "init");
+    runGit(fixtureRoot, "symbolic-ref", "HEAD", "refs/heads/main");
+    runGit(fixtureRoot, "config", "user.name", "Source Stack Test");
+    runGit(fixtureRoot, "config", "user.email", "source-stack@example.com");
+    const upstreamRef = commitFile(fixtureRoot, "base.txt", "base\n", "feat: upstream base");
+    run(fixtureRoot, upstreamRef);
+  } finally {
+    NodeFS.rmSync(fixtureRoot, { recursive: true, force: true });
+  }
+}
+
+describe("nightly source stack guard", () => {
+  it("accepts a candidate with patch-id-equivalent main commits", () => {
+    withRepository((fixtureRoot, upstreamRef) => {
+      commitFile(fixtureRoot, "one.txt", "one\n", "feat: first fork patch");
+      commitFile(fixtureRoot, "two.txt", "two\n", "fix: second fork patch");
+
+      runGit(fixtureRoot, "switch", "-c", "candidate", upstreamRef);
+      runGit(fixtureRoot, "cherry-pick", "main~1", "main");
+
+      const result = checkStack(fixtureRoot, "main", "candidate", upstreamRef);
+
+      assert.equal(result.status, 0, result.stderr);
+      assert.include(result.stdout, "contains every patch currently carried by main");
+    });
+  });
+
+  it("does not require upstream-only commits to be present in the candidate", () => {
+    withRepository((fixtureRoot, originalUpstreamRef) => {
+      runGit(fixtureRoot, "switch", "-c", "candidate");
+      const candidatePatch = commitFile(
+        fixtureRoot,
+        "fork.txt",
+        "fork\n",
+        "feat: retained fork patch",
+      );
+
+      runGit(fixtureRoot, "switch", "-c", "upstream-next", originalUpstreamRef);
+      const currentUpstreamRef = commitFile(
+        fixtureRoot,
+        "upstream.txt",
+        "upstream\n",
+        "feat: newer upstream change",
+      );
+
+      runGit(fixtureRoot, "switch", "main");
+      runGit(fixtureRoot, "merge", "--ff-only", "upstream-next");
+      runGit(fixtureRoot, "cherry-pick", candidatePatch);
+
+      const result = checkStack(fixtureRoot, "main", "candidate", currentUpstreamRef);
+
+      assert.equal(result.status, 0, result.stderr);
+    });
+  });
+
+  it("accepts a main patch that has since been absorbed by upstream", () => {
+    withRepository((fixtureRoot, originalUpstreamRef) => {
+      const mainPatch = commitFile(
+        fixtureRoot,
+        "feature.txt",
+        "fork feature\n",
+        "feat: fork patch accepted upstream",
+      );
+
+      runGit(fixtureRoot, "switch", "-c", "upstream-next", originalUpstreamRef);
+      runGit(fixtureRoot, "cherry-pick", mainPatch);
+      runGit(fixtureRoot, "commit", "--amend", "-m", "feat: upstream version of fork patch");
+      const currentUpstreamRef = runGit(fixtureRoot, "rev-parse", "HEAD");
+
+      const result = checkStack(fixtureRoot, "main", "upstream-next", currentUpstreamRef);
+
+      assert.equal(result.status, 0, result.stderr);
+    });
+  });
+
+  it("accepts equivalent patches applied on top of a newer upstream base", () => {
+    withRepository((fixtureRoot, originalUpstreamRef) => {
+      const mainPatch = commitFile(
+        fixtureRoot,
+        "fork.txt",
+        "fork\n",
+        "feat: fork patch rebased after upstream",
+      );
+
+      runGit(fixtureRoot, "switch", "-c", "upstream-next", originalUpstreamRef);
+      const currentUpstreamRef = commitFile(
+        fixtureRoot,
+        "upstream.txt",
+        "new upstream\n",
+        "feat: newer upstream base",
+      );
+      runGit(fixtureRoot, "switch", "-c", "candidate");
+      runGit(fixtureRoot, "cherry-pick", mainPatch);
+
+      const result = checkStack(fixtureRoot, "main", "candidate", currentUpstreamRef);
+
+      assert.equal(result.status, 0, result.stderr);
+    });
+  });
+
+  it("reports main patches when the candidate has no fork commits", () => {
+    withRepository((fixtureRoot, upstreamRef) => {
+      commitFile(fixtureRoot, "missing.txt", "missing\n", "fix: absent from empty candidate");
+      runGit(fixtureRoot, "branch", "candidate", upstreamRef);
+
+      const result = checkStack(fixtureRoot, "main", "candidate", upstreamRef);
+
+      assert.equal(result.status, 1);
+      assert.include(result.stderr, "fix: absent from empty candidate");
+    });
+  });
+
+  it("fails closed when git cherry fails", () => {
+    withRepository((fixtureRoot, upstreamRef) => {
+      const wrapperDirectory = NodePath.join(fixtureRoot, "bin");
+      const gitWrapper = NodePath.join(wrapperDirectory, "git");
+      NodeFS.mkdirSync(wrapperDirectory);
+      NodeFS.writeFileSync(
+        gitWrapper,
+        `#!/bin/sh
+if [ "$1" = "cherry" ]; then
+  exit 42
+fi
+PATH="$ORIGINAL_PATH" exec git "$@"
+`,
+      );
+      NodeFS.chmodSync(gitWrapper, 0o755);
+
+      const result = checkStack(fixtureRoot, "main", "main", upstreamRef, false, {
+        ...process.env,
+        ORIGINAL_PATH: process.env.PATH,
+        PATH: `${wrapperDirectory}:${process.env.PATH}`,
+      });
+
+      assert.equal(result.status, 42);
+      assert.notInclude(result.stdout, "contains every patch currently carried by main");
+    });
+  });
+
+  it("fails a stale candidate and lists each missing main commit by subject", () => {
+    withRepository((fixtureRoot, upstreamRef) => {
+      runGit(fixtureRoot, "switch", "-c", "candidate");
+      commitFile(fixtureRoot, "one.txt", "one\n", "feat: retained fork patch");
+
+      runGit(fixtureRoot, "switch", "main");
+      runGit(fixtureRoot, "merge", "--ff-only", "candidate");
+      commitFile(fixtureRoot, "two.txt", "two\n", "fix: dropped workflow guard (#91)");
+      commitFile(fixtureRoot, "three.txt", "three\n", "feat: dropped follow-up (#92)");
+
+      const result = checkStack(fixtureRoot, "main", "candidate", upstreamRef);
+
+      assert.equal(result.status, 1);
+      assert.include(result.stderr, "fix: dropped workflow guard (#91)");
+      assert.include(result.stderr, "feat: dropped follow-up (#92)");
+      assert.include(result.stderr, "Refresh the resolution from current main");
+    });
+  });
+
+  it("requires an explicit override for intentionally reshaped patches", () => {
+    withRepository((fixtureRoot, upstreamRef) => {
+      commitFile(fixtureRoot, "feature.txt", "original resolution\n", "feat: resolved feature");
+
+      runGit(fixtureRoot, "switch", "-c", "candidate", upstreamRef);
+      commitFile(fixtureRoot, "feature.txt", "reshaped resolution\n", "feat: resolved feature");
+
+      const rejected = checkStack(fixtureRoot, "main", "candidate", upstreamRef);
+      const allowed = checkStack(fixtureRoot, "main", "candidate", upstreamRef, true);
+
+      assert.equal(rejected.status, 1);
+      assert.equal(allowed.status, 0, allowed.stderr);
+      assert.include(allowed.stderr, "feat: resolved feature");
+      assert.include(allowed.stderr, "Missing main patches explicitly allowed");
+    });
+  });
+});