diff --git a/agents/implementer-agent.md b/agents/implementer-agent.md
index ecb441e..a1b066a 100644
--- a/agents/implementer-agent.md
+++ b/agents/implementer-agent.md
@@ -146,6 +146,11 @@ Move content from one file to another.
 
 Append to: `~/.claude/config-audit/sessions/{session-id}/implementation-log.md`
 
+**Append discipline (shared log):** other implementer agents may be writing this
+log concurrently. ALWAYS append your entry with a Bash `>>` heredoc;
+NEVER use the Write or Edit tool on the log file — a full-file Write silently
+clobbers entries other agents appended after you read the file.
+
 ### Success
 
 ```markdown
diff --git a/commands/implement.md b/commands/implement.md
index 5f13d51..f2ff97a 100644
--- a/commands/implement.md
+++ b/commands/implement.md
@@ -78,6 +78,8 @@ Agent(subagent_type: "config-audit:implementer-agent")
     fields from the action plan (the planner already rendered them) —
     do not re-derive severity prose. Append result to:
     ~/.claude/config-audit/sessions/{session-id}/implementation-log.md
+    Append with Bash `>>` (heredoc) — NEVER the Write tool on this log;
+    parallel agents share it and a full-file Write clobbers their entries.
 ```
 
 Show progress between groups using the humanized titles already present in the action plan:
diff --git a/tests/commands/implement-log-append.test.mjs b/tests/commands/implement-log-append.test.mjs
new file mode 100644
index 0000000..09f0d6c
--- /dev/null
+++ b/tests/commands/implement-log-append.test.mjs
@@ -0,0 +1,41 @@
+/**
+ * M-BUG-20 — shared implementation-log clobbering under parallel agents.
+ *
+ * implement.md step 4 spawns implementer agents in parallel batches, and every
+ * agent appends its result to the SAME implementation-log.md. Dogfooding
+ * (2026-07-17, throwaway linkedin-posts copy) showed agents satisfying
+ * "Append result to:" with a full-file Write: each agent read the log, added
+ * its entry, and wrote the whole file back — the last writer silently
+ * clobbered 4 of 6 entries.
+ *
+ * Contract: both the command template and the agent prompt must pin the append
+ * mechanism — Bash `>>`, never the Write/Edit tool — on the shared log.
+ */
+
+import { test } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { readFile } from 'node:fs/promises';
+import { resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const ROOT = resolve(__dirname, '..', '..');
+
+const APPEND_MECHANISM_REGEX = />>/;
+const FORBID_WRITE_TOOL_REGEX = /never[^.\n]*\bwrite\b[^.\n]*tool|\bwrite\b[^.\n]*tool[^.\n]*never/i;
+
+test('implement.md: agent-spawn template pins Bash >> append on the shared log', async () => {
+  const content = await readFile(resolve(ROOT, 'commands', 'implement.md'), 'utf-8');
+  assert.match(content, APPEND_MECHANISM_REGEX,
+    'implement.md must instruct appending to implementation-log.md with Bash >>');
+  assert.match(content, FORBID_WRITE_TOOL_REGEX,
+    'implement.md must forbid the Write tool on the shared implementation log');
+});
+
+test('implementer-agent.md: output section pins Bash >> append and forbids Write tool on the log', async () => {
+  const content = await readFile(resolve(ROOT, 'agents', 'implementer-agent.md'), 'utf-8');
+  assert.match(content, APPEND_MECHANISM_REGEX,
+    'implementer-agent.md must instruct appending to the log with Bash >>');
+  assert.match(content, FORBID_WRITE_TOOL_REGEX,
+    'implementer-agent.md must forbid the Write tool on the shared implementation log');
+});