fix: prohibit Co-Authored-By trailers unless explicitly requested

CONTRIBUTING.md

@@ -148,6 +148,8 @@ refactor: simplify DAG executor
 test: add unit tests for cost router
 ```
 
+Do not add `Co-Authored-By` or any attribution trailer to commits or PRs unless a maintainer explicitly asks for it. Do not infer authorship from the model you are using.
+
 ### Code Style
 
 - TypeScript strict mode — `tsconfig.json` sets `"strict": true`

assets/agents/nexus-architect.md

@@ -43,6 +43,10 @@ You are a senior software architect. You design systems that are scalable, resil
 - Synchronous chains that create tight coupling
 - Over-engineering simple problems (YAGNI)
 
+## Commits
+
+- NEVER add `Co-Authored-By` or any attribution trailer (e.g. `Co-Authored-By: Claude`) to a commit or PR unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.
+
 ## Output Contract
 
 Return a written architecture direction, not code. It must contain:

assets/agents/nexus-coder.md

@@ -48,6 +48,10 @@ You are a senior software engineer who writes clean, maintainable, production-re
 - Tests covering happy path, edge cases, and error paths
 - Brief inline comments for complex logic (why, not what)
 
+## Commits
+
+- NEVER add `Co-Authored-By` or any attribution trailer (e.g. `Co-Authored-By: Claude`) to a commit or PR unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.
+
 ## Output Contract
 
 Return what you changed and how it was verified. It must contain:

assets/agents/nexus-documenter.md

@@ -49,6 +49,10 @@ You are a technical writer who creates documentation that developers actually wa
 - TODO/FIXME/HACK comments are tracked and explained
 - Changelog follows semantic versioning with clear descriptions
 
+## Commits
+
+- NEVER add `Co-Authored-By` or any attribution trailer (e.g. `Co-Authored-By: Claude`) to a commit or PR unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.
+
 ## Output Contract
 
 Documentation that is true of the code as it is now. It must contain:

assets/agents/nexus-explorer.md

@@ -39,6 +39,10 @@ You are a code archaeologist. You navigate unknown codebases efficiently and bui
 - Report uncertainty explicitly — don't guess about unexamined code
 - Cite specific file paths and line numbers for all findings
 
+## Commits
+
+- NEVER add `Co-Authored-By` or any attribution trailer (e.g. `Co-Authored-By: Claude`) to a commit or PR unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.
+
 ## Output Contract
 
 A map, with evidence. It must contain:

assets/agents/nexus-orchestrator.md

@@ -206,6 +206,7 @@ When code changes are needed, follow this workflow:
 - One logical change per commit
 - Imperative mood in commit message
 - Reference issues if applicable
+- NEVER add `Co-Authored-By` or any attribution trailer (e.g. `Co-Authored-By: Claude`) to commits or PR descriptions unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.
 
 ### 4. Pull Request
 - Create PR with descriptive title and body

assets/agents/nexus-reviewer.md

@@ -52,6 +52,10 @@ For each finding:
 - Be thorough — check for issues the author might have missed
 - Prioritize — Critical/High issues first, then Medium/Low
 
+## Commits
+
+- NEVER add `Co-Authored-By` or any attribution trailer (e.g. `Co-Authored-By: Claude`) to a commit or PR unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.
+
 ## Output Contract
 
 Findings, ordered by severity, and nothing else. Each finding must carry:

assets/agents/nexus-tester.md

@@ -47,6 +47,10 @@ You are a QA engineer who writes tests that catch real bugs, not just increase c
 - Edge case coverage alongside happy path
 - Mock/stub strategy that doesn't hide real bugs
 
+## Commits
+
+- NEVER add `Co-Authored-By` or any attribution trailer (e.g. `Co-Authored-By: Claude`) to a commit or PR unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.
+
 ## Output Contract
 
 Tests plus evidence they run. It must contain:

src/index.ts

@@ -896,6 +896,7 @@ export function buildGitFlowConventionSection(
 
   lines.push(
     '- **Never commit, push, or merge on your own initiative.** Report the change you made and let the user decide. If the user explicitly asks you to commit, push or merge, that is the one case where you do.',
+    '- **No attribution trailers.** Never add `Co-Authored-By` or any similar trailer (e.g. `Co-Authored-By: Claude`) to a commit or PR unless the user explicitly asks for it in this conversation. Do not infer authorship from the model in use.',
     '',
     'You can check where things stand with `nexus.git.check`. It only reports — it never changes anything and never refuses.',
     '',

test/agent-definitions.test.ts

@@ -154,4 +154,44 @@ describe('each role states an output contract and when to escalate', () => {
       expect(content).toMatch(/^## When to Escalate$/m)
     }
   )
-})
+})
+
+describe('commit-capable prompts prohibit attribution trailers', () => {
+  // Roles whose frontmatter grants `shell` can run `git commit`; the
+  // orchestrator opens PRs. Models hallucinate trailers such as
+  // "Co-Authored-By: Claude" even when the model is not Claude, so the rule is
+  // pinned per prompt. Only designer denies `shell`; reviewer and explorer
+  // deny only `edit`, and OpenCode's permission resolution falls through to a
+  // catch-all allow for them — they CAN run `git commit`, so they carry the
+  // rule too. Designer's protection is the appended git-convention section
+  // (`buildGitFlowConventionSection`), covered in `test/git-flow.test.ts`.
+  const COMMIT_CAPABLE = ['nexus-coder.md', 'nexus-tester.md', 'nexus-documenter.md', 'nexus-architect.md', 'nexus-reviewer.md', 'nexus-explorer.md', ORCHESTRATOR]
+
+  it.each(COMMIT_CAPABLE)(
+    '%s forbids Co-Authored-By and similar trailers unless the user asks',
+    (name) => {
+      // One anchored regex, not three loose substrings: the substring version
+      // passes for an INVERTED rule ("never add ... unless ... Do not
+      // infer ..."), because each fragment is still present somewhere in the
+      // file. The anchor ties the fragments together in the order the rule
+      // states them, so an inversion fails here.
+      const content = readAgent(name)
+      expect(content).toMatch(
+        /NEVER add `Co-Authored-By`[\s\S]{0,120}unless the user explicitly asks for it in this conversation\. Do not infer authorship from the model in use\./
+      )
+    }
+  )
+
+  it('every agent that can shell carries the attribution rule', () => {
+    // OpenCode's permission resolution falls through to a catch-all allow, so
+    // an agent that never names `shell` can still run `git commit`. Stated
+    // that way round on purpose: unless the frontmatter explicitly denies
+    // `shell`, the prompt must carry the rule. This is the invariant that
+    // would have caught reviewer and explorer missing it.
+    for (const name of [...SPAWNABLE_ROLES.map((r) => `nexus-${r}.md`), ORCHESTRATOR]) {
+      const frontmatter = frontmatterOf(readAgent(name))
+      const shellDenied = /action:\s*shell[\s\S]*?effect:\s*deny/.test(frontmatter)
+      if (!shellDenied) expect(readAgent(name)).toMatch(/Co-Authored-By/)
+    }
+  })
+})

test/git-flow.test.ts

@@ -757,6 +757,32 @@ describe('the agent-markdown convention', () => {
     expect(section).toContain('Never commit, push, or merge on your own initiative')
   })
 
+  it('never emits an attribution-trailer clause with any sub-key off', () => {
+    // The no-Co-Authored-By rule is unconditional for the same reason the
+    // unprompted-commit rule is: models hallucinate trailers such as
+    // "Co-Authored-By: Claude" even when the model is not Claude, and no
+    // gitFlow key is a knob for authorship. Pinned so a refactor that drops it
+    // behind a key fails here rather than in a polluted commit history.
+    //
+    // `gitFlow.enabled: false` is deliberately NOT one of the configs below:
+    // that is the master switch, and it removes the whole convention section by
+    // design — `src/index.ts` gates `buildGitFlowConventionSection` on
+    // `gitFlowResolved.active`. What this pins is that no SUB-key can disable
+    // the clause.
+    for (const config of [
+      ALL_ON,
+      { ...ALL_ON, requireBranch: false },
+      { ...ALL_ON, conventionalCommits: false },
+      { ...ALL_ON, prBeforeMerge: false },
+      { ...ALL_ON, requireBranch: false, conventionalCommits: false, prBeforeMerge: false },
+    ]) {
+      const section = buildGitFlowConventionSection(config, { isRepo: true, repoRoot: '/x', repoId: '/x/.git', branch: 'feat/y', detached: false, hasRemote: true, remote: 'origin', known: true })
+      expect(section).toContain('No attribution trailers')
+      expect(section).toContain('Co-Authored-By')
+      expect(section).toContain('unless the user explicitly asks')
+    }
+  })
+
   it('tells the agent the section can be turned off, and names the check as read-only', () => {
     const section = buildGitFlowConventionSection(ALL_ON, { isRepo: true, repoRoot: '/x', repoId: '/x/.git', branch: 'main', detached: false, hasRemote: false, remote: null, known: true })
     expect(section).toContain('gitFlow.enabled')