diff --git a/scripts/validate-gh-aw-cast.mjs b/scripts/validate-gh-aw-cast.mjs
index aea594a72..ff02ea152 100644
--- a/scripts/validate-gh-aw-cast.mjs
+++ b/scripts/validate-gh-aw-cast.mjs
@@ -32,6 +32,12 @@ const BUILTIN_DISPLAY_NAMES = {
 const REQUIRED_BUILTIN_CHARTERS = REQUIRED_BUILTIN_IDS
   .map((id) => `.squad/agents/${id}/charter.md`)
   .sort();
+// The canonical resource a gh-aw "Materialize canonical built-in support
+// agents" step copies from verbatim before the agent ever runs. The final
+// emitted `.squad/agents/{id}/charter.md` must remain byte-identical to it —
+// this is the only way to prove a `squad init` or agent rewrite never
+// clobbered the materialized built-in after the deterministic copy step.
+const BUILTIN_CANONICAL_DIR = '.github/workflows/shared/builtins';
 const BUILTIN_SECTION_HEADING = '## Built-in Support Agents';
 const CAST_SOURCES_HEADING = '## Cast sources';
 const BUILTIN_NAME_ROW_PATTERN = /^\|\s*(Scribe|Ralph|Rai|Fact Checker)\s*\|/gmi;
@@ -69,6 +75,45 @@ function readText(root, relativePath) {
   return readFileSync(join(root, ...relativePath.split('/')), 'utf8').replace(/\r\n/g, '\n');
 }
 
+/** Raw file bytes, with no text normalization, for true byte-for-byte comparison. */
+function readBytes(root, relativePath) {
+  return readFileSync(join(root, ...relativePath.split('/')));
+}
+
+/**
+ * Every materialized built-in charter must remain byte-for-byte identical to
+ * the canonical resource shipped with the workflow. A deterministic step
+ * copies the canonical resource verbatim before the agent runs; if `squad
+ * init` or the Cast agent later rewrites, paraphrases, or reinterprets one of
+ * these files, this is the only check that catches the divergence.
+ */
+function validateBuiltinCharterFidelity(root, errors) {
+  for (const id of REQUIRED_BUILTIN_IDS) {
+    const canonicalPath = `${BUILTIN_CANONICAL_DIR}/${id}-charter.md`;
+    const materializedPath = `.squad/agents/${id}/charter.md`;
+    let canonicalBytes;
+    try {
+      canonicalBytes = readBytes(root, canonicalPath);
+    } catch (error) {
+      errors.push(`builtin: canonical resource ${canonicalPath} for "${id}" is missing or unreadable (${error.message})`);
+      continue;
+    }
+    let materializedBytes;
+    try {
+      materializedBytes = readBytes(root, materializedPath);
+    } catch (error) {
+      errors.push(`builtin: materialized charter ${materializedPath} for "${id}" is missing or unreadable (${error.message})`);
+      continue;
+    }
+    if (!canonicalBytes.equals(materializedBytes)) {
+      errors.push(
+        `builtin: ${materializedPath} is not byte-identical to the canonical resource `
+        + `${canonicalPath} — built-in charters must never be regenerated, edited, or reinterpreted`,
+      );
+    }
+  }
+}
+
 function normalizePayloadPath(value) {
   if (
     typeof value !== 'string'
@@ -333,6 +378,8 @@ export function validateCastTree({ root, payloadPath }) {
     }
   }
 
+  validateBuiltinCharterFidelity(root, errors);
+
   let team = '';
   let routing = '';
   let coordinator = '';
diff --git a/test/gh-aw-cast-validator.test.ts b/test/gh-aw-cast-validator.test.ts
index ef6d3e316..64106e392 100644
--- a/test/gh-aw-cast-validator.test.ts
+++ b/test/gh-aw-cast-validator.test.ts
@@ -26,6 +26,13 @@ const builtins = [
   { id: 'fact-checker', name: 'Fact Checker' },
 ];
 
+/** Byte-for-byte canonical content of a built-in charter, as shipped with the workflow. */
+function builtinCanonicalContent(id: string): Buffer {
+  return readFileSync(join(process.cwd(), 'workflows', 'shared', 'builtins', `${id}-charter.md`));
+}
+
+const builtinCanonicalRelativePath = '.github/workflows/shared/builtins';
+
 const corePayload = [
   '.squad/team.md',
   '.squad/routing.md',
@@ -38,7 +45,7 @@ const corePayload = [
   'meet-the-squad.md',
 ];
 
-function write(root: string, path: string, content: string): void {
+function write(root: string, path: string, content: string | Buffer): void {
   const fullPath = join(root, ...path.split('/'));
   mkdirSync(dirname(fullPath), { recursive: true });
   writeFileSync(fullPath, content, 'utf8');
@@ -171,7 +178,12 @@ function createFixture(): { root: string; payload: string; runnerTemp: string }
     write(root, `.squad/agents/${member.id}/charter.md`, `# ${member.name} — ${member.role}\n`);
   }
   for (const builtin of builtins) {
-    write(root, `.squad/agents/${builtin.id}/charter.md`, `# ${builtin.name}\n`);
+    const canonical = builtinCanonicalContent(builtin.id);
+    // Mirrors the gh-aw resource materialization: the canonical charter is
+    // installed under .github/workflows/shared/builtins/, then copied
+    // verbatim to .squad/agents/{id}/charter.md by a deterministic step.
+    write(root, `${builtinCanonicalRelativePath}/${builtin.id}-charter.md`, canonical);
+    write(root, `.squad/agents/${builtin.id}/charter.md`, canonical);
   }
   write(root, '.github/agents/squad.agent.md', coordinatorMarkdown());
   write(root, 'meet-the-squad.md', '# Meet the Squad\n');
@@ -417,7 +429,7 @@ describe('GH-AW Cast final-tree validator', () => {
     const result = runValidatorCommand(fixture);
     expect(result.status).not.toBe(0);
     expect(result.stderr).toMatch(
-      /Cast validator SHA-256 mismatch: expected 5b8e1432de63c78488ee7ff6da4e67ea0c91044582109f678153c231704a0097, got [a-f0-9]{64}\./,
+      /Cast validator SHA-256 mismatch: expected f0c79694d9832c53070f059d4bff181a8ccd857e1be49d24b8d5b72ed8887251, got [a-f0-9]{64}\./,
     );
     expect(result.stdout).not.toContain('Cast validation passed.');
     expect(authorizesPullRequest(result)).toBe(false);
@@ -625,6 +637,49 @@ describe('GH-AW Cast final-tree validator', () => {
     expect(result.stderr).toContain('watcher');
   });
 
+  it('rejects a built-in charter that has been regenerated or edited (byte mismatch vs. canonical resource)', () => {
+    const fixture = createFixture();
+    write(
+      fixture.root,
+      '.squad/agents/ralph/charter.md',
+      `${builtinCanonicalContent('ralph').toString('utf8')}\n<!-- reinterpreted by the Cast agent -->\n`,
+    );
+    const result = validate(fixture.root, fixture.payload);
+    expect(result.status).toBe(1);
+    expect(result.stderr).toMatch(
+      /builtin: \.squad\/agents\/ralph\/charter\.md is not byte-identical to the canonical resource \.github\/workflows\/shared\/builtins\/ralph-charter\.md/i,
+    );
+  });
+
+  it('rejects a built-in charter with a trivial byte-level divergence (trailing newline) from the canonical resource', () => {
+    const fixture = createFixture();
+    write(fixture.root, '.squad/agents/scribe/charter.md', `${builtinCanonicalContent('scribe').toString('utf8')}\n`);
+    const result = validate(fixture.root, fixture.payload);
+    expect(result.status).toBe(1);
+    expect(result.stderr).toMatch(/builtin: \.squad\/agents\/scribe\/charter\.md is not byte-identical/i);
+  });
+
+  it('rejects a Cast tree whose canonical built-in resource is missing from .github/workflows/shared/builtins', () => {
+    const fixture = createFixture();
+    rmSync(join(fixture.root, builtinCanonicalRelativePath, 'fact-checker-charter.md'), { force: true });
+    const result = validate(fixture.root, fixture.payload);
+    expect(result.status).toBe(1);
+    expect(result.stderr).toMatch(
+      /builtin: canonical resource \.github\/workflows\/shared\/builtins\/fact-checker-charter\.md for "fact-checker" is missing or unreadable/i,
+    );
+  });
+
+  it('accepts a built-in charter that is byte-identical to the canonical resource for all four built-ins', () => {
+    const fixture = createFixture();
+    const result = validate(fixture.root, fixture.payload);
+    expect(result.status, result.stderr).toBe(0);
+    for (const builtin of builtins) {
+      expect(
+        readFileSync(join(fixture.root, '.squad', 'agents', builtin.id, 'charter.md')),
+      ).toEqual(builtinCanonicalContent(builtin.id));
+    }
+  });
+
   it('rejects a built-in registered as an active specialist in the casting registry', () => {
     const fixture = createFixture();
     write(fixture.root, '.squad/casting/registry.json', JSON.stringify({
diff --git a/test/gh-aw-quality.test.ts b/test/gh-aw-quality.test.ts
index ef0fb0d1b..ecf70cf15 100644
--- a/test/gh-aw-quality.test.ts
+++ b/test/gh-aw-quality.test.ts
@@ -1506,14 +1506,67 @@ describe('gh-aw: compiled workflow shell input security contract', () => {
       '--payload "${GITHUB_WORKSPACE:?}/.github/workflows/squad-cast-payload.json"',
     );
     expect(normalizedRunnerStep).not.toContain('RUNNER_TEMP');
-    expect(normalizedRunnerStep).toContain('validator_expected_sha256="5b8e1432de63c78488ee7ff6da4e67ea0c91044582109f678153c231704a0097"');
+    expect(normalizedRunnerStep).toContain('validator_expected_sha256="f0c79694d9832c53070f059d4bff181a8ccd857e1be49d24b8d5b72ed8887251"');
     expect(normalizedRunnerStep).toContain("outcome: 'cast_failure'");
     expect(normalizedRunnerStep).toContain('chmod 500 "$validator_runner"');
+    // Prepared as a pre-agent-step (see the built-in fidelity ordering test below):
+    // it must run AFTER the base-branch/ambient restores that can reintroduce a
+    // stale committed .squad snapshot, and still before the agent turn begins.
+    expect(compiled.indexOf('name: Restore inline skills from activation artifact')).toBeLessThan(
+      compiled.indexOf('name: Prepare deterministic Cast validator runner'),
+    );
     expect(compiled.indexOf('name: Prepare deterministic Cast validator runner')).toBeLessThan(
-      compiled.indexOf('name: Restore inline skills from activation artifact'),
+      compiled.indexOf('name: Execute GitHub Copilot CLI'),
     );
   }, 20000);
 
+  it('materializes built-in charters after init and every late restore, and before the Cast agent turn', () => {
+    const compiled = lockText();
+
+    // Job-level guarantee: the agent job cannot start until the activation
+    // job (which runs `squad init --preset default` when no cast exists yet)
+    // has completed.
+    expect(compiled).toMatch(/\n {2}agent:\n {4}needs: activation\n/);
+    const activationJobIndex = compiled.indexOf('\n  activation:\n');
+    const agentJobIndex = compiled.indexOf('\n  agent:\n');
+    expect(activationJobIndex).toBeGreaterThan(-1);
+    expect(agentJobIndex).toBeGreaterThan(activationJobIndex);
+
+    const initIndex = compiled.indexOf('name: Initialize Squad team');
+    const materializeIndex = compiled.indexOf('name: Materialize canonical built-in support agents');
+    expect(initIndex).toBeGreaterThan(-1);
+    expect(materializeIndex).toBeGreaterThan(initIndex);
+
+    // Materialization is a pre-agent-step: it must run strictly after every
+    // restore step that can reintroduce a stale committed `.squad/` snapshot
+    // late in the job (base-branch restore, both ambient-folder restores,
+    // inline sub-agent/skill restores), and strictly before the Cast agent
+    // turn (Copilot CLI execution) starts.
+    for (const priorRestore of [
+      'name: Restore Squad state from activation artifact',
+      'name: Restore agent config folders from base branch',
+      'name: Restore inline sub-agents from activation artifact',
+      'name: Restore inline skills from activation artifact',
+    ]) {
+      const restoreIndex = compiled.indexOf(priorRestore);
+      expect(restoreIndex, `${priorRestore} must be present`).toBeGreaterThan(-1);
+      expect(materializeIndex, `${priorRestore} must precede materialization`).toBeGreaterThan(restoreIndex);
+    }
+    // Both occurrences of the unconditional ambient-folder restore (gh-aw emits
+    // it once after the activation-artifact download and again after PR
+    // checkout) must precede materialization too.
+    const ambientRestoreIndices = [...compiled.matchAll(/name: Restore ambient folders from activation artifact/g)]
+      .map((match) => match.index ?? -1);
+    expect(ambientRestoreIndices.length).toBeGreaterThanOrEqual(2);
+    for (const restoreIndex of ambientRestoreIndices) {
+      expect(materializeIndex).toBeGreaterThan(restoreIndex);
+    }
+
+    const agentExecutionIndex = compiled.indexOf('name: Execute GitHub Copilot CLI');
+    expect(agentExecutionIndex).toBeGreaterThan(-1);
+    expect(agentExecutionIndex).toBeGreaterThan(materializeIndex);
+  }, 20000);
+
   it('keeps the v0.87.10 completion hook neutral when a custom safe job fails', () => {
     const compiled = lockText();
     const completionStep = compiled.match(
diff --git a/workflows/shared/squad-cast-validator.mjs b/workflows/shared/squad-cast-validator.mjs
index aea594a72..ff02ea152 100644
--- a/workflows/shared/squad-cast-validator.mjs
+++ b/workflows/shared/squad-cast-validator.mjs
@@ -32,6 +32,12 @@ const BUILTIN_DISPLAY_NAMES = {
 const REQUIRED_BUILTIN_CHARTERS = REQUIRED_BUILTIN_IDS
   .map((id) => `.squad/agents/${id}/charter.md`)
   .sort();
+// The canonical resource a gh-aw "Materialize canonical built-in support
+// agents" step copies from verbatim before the agent ever runs. The final
+// emitted `.squad/agents/{id}/charter.md` must remain byte-identical to it —
+// this is the only way to prove a `squad init` or agent rewrite never
+// clobbered the materialized built-in after the deterministic copy step.
+const BUILTIN_CANONICAL_DIR = '.github/workflows/shared/builtins';
 const BUILTIN_SECTION_HEADING = '## Built-in Support Agents';
 const CAST_SOURCES_HEADING = '## Cast sources';
 const BUILTIN_NAME_ROW_PATTERN = /^\|\s*(Scribe|Ralph|Rai|Fact Checker)\s*\|/gmi;
@@ -69,6 +75,45 @@ function readText(root, relativePath) {
   return readFileSync(join(root, ...relativePath.split('/')), 'utf8').replace(/\r\n/g, '\n');
 }
 
+/** Raw file bytes, with no text normalization, for true byte-for-byte comparison. */
+function readBytes(root, relativePath) {
+  return readFileSync(join(root, ...relativePath.split('/')));
+}
+
+/**
+ * Every materialized built-in charter must remain byte-for-byte identical to
+ * the canonical resource shipped with the workflow. A deterministic step
+ * copies the canonical resource verbatim before the agent runs; if `squad
+ * init` or the Cast agent later rewrites, paraphrases, or reinterprets one of
+ * these files, this is the only check that catches the divergence.
+ */
+function validateBuiltinCharterFidelity(root, errors) {
+  for (const id of REQUIRED_BUILTIN_IDS) {
+    const canonicalPath = `${BUILTIN_CANONICAL_DIR}/${id}-charter.md`;
+    const materializedPath = `.squad/agents/${id}/charter.md`;
+    let canonicalBytes;
+    try {
+      canonicalBytes = readBytes(root, canonicalPath);
+    } catch (error) {
+      errors.push(`builtin: canonical resource ${canonicalPath} for "${id}" is missing or unreadable (${error.message})`);
+      continue;
+    }
+    let materializedBytes;
+    try {
+      materializedBytes = readBytes(root, materializedPath);
+    } catch (error) {
+      errors.push(`builtin: materialized charter ${materializedPath} for "${id}" is missing or unreadable (${error.message})`);
+      continue;
+    }
+    if (!canonicalBytes.equals(materializedBytes)) {
+      errors.push(
+        `builtin: ${materializedPath} is not byte-identical to the canonical resource `
+        + `${canonicalPath} — built-in charters must never be regenerated, edited, or reinterpreted`,
+      );
+    }
+  }
+}
+
 function normalizePayloadPath(value) {
   if (
     typeof value !== 'string'
@@ -333,6 +378,8 @@ export function validateCastTree({ root, payloadPath }) {
     }
   }
 
+  validateBuiltinCharterFidelity(root, errors);
+
   let team = '';
   let routing = '';
   let coordinator = '';
diff --git a/workflows/squad.md b/workflows/squad.md
index 7cae87d25..44427433e 100644
--- a/workflows/squad.md
+++ b/workflows/squad.md
@@ -54,7 +54,11 @@ tools:
   github:
     mode: gh-proxy
     toolsets: [default]
-steps:
+# pre-agent-steps (not steps:): runs after gh-aw's native base-branch/ambient
+# restores that can reintroduce a stale committed .squad/ snapshot late in the
+# job, so this stays the last writer of the four built-in charters before the
+# agent turn begins.
+pre-agent-steps:
   - name: Materialize canonical built-in support agents
     shell: bash
     run: |
@@ -121,7 +125,7 @@ steps:
       fi
       validator_script="$(cd "$(dirname "$validator_script")" && pwd -P)/$(basename "$validator_script")"
 
-      validator_expected_sha256="5b8e1432de63c78488ee7ff6da4e67ea0c91044582109f678153c231704a0097"
+      validator_expected_sha256="f0c79694d9832c53070f059d4bff181a8ccd857e1be49d24b8d5b72ed8887251"
       : > "$stderr_file"
       validator_actual_sha256="$(
         node -e 'const c=require("node:crypto"),f=require("node:fs");process.stdout.write(c.createHash("sha256").update(f.readFileSync(process.argv[1])).digest("hex"))' \