diff --git a/askcc/definitions.py b/askcc/definitions.py
index 8f0977c..db19c59 100644
--- a/askcc/definitions.py
+++ b/askcc/definitions.py
@@ -181,9 +181,19 @@
 Implementation:
 - Read the issue's planned implementation (in comments) before writing code.
 - Conform to the project's existing style, structure, and conventions.
-- Write tests for every new or changed behavior.
 - Make focused, minimal changes — do not refactor unrelated code.
 
+Testing methodology — red/green TDD (non-negotiable):
+- RED: write a failing test that captures the required behavior. Run it and confirm it fails \
+for the right reason. Paste the failing output into your working notes.
+- GREEN: write the minimum code needed to pass the test. No extra features, no speculative \
+abstractions, no "while I'm here" changes.
+- REFACTOR: only if there is clear duplication or unclear naming. Keep tests green throughout.
+- Tests must assert on observable inputs/outputs or side effects — never by re-invoking the \
+implementation's own logic to compute the expected value.
+- Do NOT proceed from RED to GREEN without a confirmed failing run.
+- Commit tests and implementation together.
+
 Decisions:
 - When you make a judgment call not specified in the plan, document it as: \
 "DECISION: <what> because <why>."
@@ -202,6 +212,8 @@
   ```
 
 Anti-rationalization — do not take these shortcuts:
+- "I'll write the test after, it's faster" — Tests written after implementation verify the code, \
+not the requirement. Always RED before GREEN.
 - "Tests aren't needed for this small change" — Small changes cause the majority of regressions. \
 Every behavioral change requires a test.
 - "I'll skip linting, the CI will catch it" — Catching errors locally is cheaper than a failed CI round-trip. \
diff --git a/tests/test_askcc.py b/tests/test_askcc.py
index 8862cca..b3bf0ac 100644
--- a/tests/test_askcc.py
+++ b/tests/test_askcc.py
@@ -9,7 +9,13 @@
 import pytest
 
 from askcc.cli import main
-from askcc.definitions import AGENT_CONFIGS, AgentAction, AgentConfig, SupportedLanguage
+from askcc.definitions import (
+    AGENT_CONFIGS,
+    DEVELOP_AGENT_PROMPT,
+    AgentAction,
+    AgentConfig,
+    SupportedLanguage,
+)
 from askcc.functions import (
     CheckResult,
     VerificationResult,
@@ -229,6 +235,37 @@ def test_raises_on_missing_required_variable(self, tmp_path: Path, monkeypatch:
             load_agent_config(AgentAction.PLAN)
 
 
+class TestDevelopPromptTddContent:
+    def test_includes_red_green_tdd_block(self):
+        assert "Testing methodology — red/green TDD (non-negotiable):" in DEVELOP_AGENT_PROMPT
+
+    def test_includes_red_phase_with_failure_confirmation(self):
+        assert "RED:" in DEVELOP_AGENT_PROMPT
+        assert "confirm it fails" in DEVELOP_AGENT_PROMPT
+
+    def test_includes_green_phase_with_minimum_code(self):
+        assert "GREEN:" in DEVELOP_AGENT_PROMPT
+        assert "minimum code" in DEVELOP_AGENT_PROMPT
+
+    def test_includes_refactor_phase(self):
+        assert "REFACTOR:" in DEVELOP_AGENT_PROMPT
+
+    def test_includes_phase_gate_blocking_red_to_green(self):
+        assert "Do NOT proceed from RED to GREEN without a confirmed failing run." in DEVELOP_AGENT_PROMPT
+
+    def test_includes_behavioral_assertion_rule(self):
+        assert "observable inputs/outputs or side effects" in DEVELOP_AGENT_PROMPT
+
+    def test_requires_committing_tests_with_implementation(self):
+        assert "Commit tests and implementation together." in DEVELOP_AGENT_PROMPT
+
+    def test_anti_rationalization_includes_tests_after_entry(self):
+        assert "I'll write the test after, it's faster" in DEVELOP_AGENT_PROMPT
+
+    def test_removes_legacy_write_tests_line(self):
+        assert "Write tests for every new or changed behavior" not in DEVELOP_AGENT_PROMPT
+
+
 class TestWritePromptContent:
     def test_writes_file_with_correct_name(self):
         filepath = write_prompt_content("plan", "monkut", "askcc-cli", 42, "issue content here")