diff --git a/src/foundation/log.c b/src/foundation/log.c
index 3e5f90c43..fd916524a 100644
--- a/src/foundation/log.c
+++ b/src/foundation/log.c
@@ -66,11 +66,9 @@ parse_format:;
         return;
     }
 
-    const char *env = getenv("ENVIRONMENT");
-    const char *cloud_run = getenv("K_SERVICE");
-    if ((env && strcmp(env, "production") == 0) || (cloud_run && cloud_run[0] != '\0')) {
-        cbm_log_set_format(CBM_LOG_FORMAT_JSON);
-    }
+    /* Format is intentionally explicit-only. Logs stay local to stderr and the
+     * optional in-process sink; deployment environment variables must not
+     * silently change the operator-selected output shape. */
 }
 
 void cbm_log_set_sink(cbm_log_sink_fn fn) {
diff --git a/src/foundation/log.h b/src/foundation/log.h
index 74c68d45a..7f247f086 100644
--- a/src/foundation/log.h
+++ b/src/foundation/log.h
@@ -4,7 +4,7 @@
  * Design:
  *   - All output goes to stderr (stdout is reserved for MCP JSON-RPC)
  *   - Structured text format: "level=info msg=pass.timing pass=defs elapsed_ms=42"
- *   - Optional JSON format for production collectors / Cloud Logging
+ *   - Optional JSON format for local structured parsing
  *   - Levels: DEBUG, INFO, WARN, ERROR
  *   - Level filtering at runtime via cbm_log_set_level() or the
  *     CBM_LOG_LEVEL env var (see cbm_log_init_from_env)
@@ -40,9 +40,8 @@ typedef enum {
  * the numeric equivalents 0..4 matching CBMLogLevel. Unknown, empty, or
  * unset values leave the level unchanged (fail-open).
  *
- * Also applies CBM_LOG_FORMAT=text|json. If unset, JSON is auto-enabled for
- * production / Cloud Run style environments (ENVIRONMENT=production or
- * K_SERVICE present). Call once at startup before any threads or log lines. */
+ * Also applies CBM_LOG_FORMAT=text|json. If unset, the current format is left
+ * unchanged. Call once at startup before any threads or log lines. */
 void cbm_log_init_from_env(void);
 
 /* Set minimum log level (default: INFO). */
diff --git a/tests/test_log.c b/tests/test_log.c
index 8165d8967..5a1fdb7aa 100644
--- a/tests/test_log.c
+++ b/tests/test_log.c
@@ -197,6 +197,19 @@ TEST(log_format_from_env) {
     PASS();
 }
 
+TEST(log_format_is_explicit_only) {
+    cbm_unsetenv("CBM_LOG_FORMAT");
+    cbm_setenv("ENVIRONMENT", "production", 1);
+    cbm_setenv("K_SERVICE", "cbm", 1);
+    cbm_log_set_format(CBM_LOG_FORMAT_TEXT);
+    cbm_log_init_from_env();
+    ASSERT_EQ(cbm_log_get_format(), CBM_LOG_FORMAT_TEXT);
+
+    cbm_unsetenv("ENVIRONMENT");
+    cbm_unsetenv("K_SERVICE");
+    PASS();
+}
+
 /* CBM_LOG_LEVEL parsing — distilled from #414 (closes #413). */
 TEST(log_level_from_env_textual) {
     cbm_setenv("CBM_LOG_LEVEL", "error", 1);
@@ -281,6 +294,7 @@ SUITE(log) {
     RUN_TEST(log_sink_tee_keeps_stderr);
     RUN_TEST(log_operational_helpers);
     RUN_TEST(log_format_from_env);
+    RUN_TEST(log_format_is_explicit_only);
     RUN_TEST(log_level_from_env_textual);
     RUN_TEST(log_level_from_env_numeric);
     RUN_TEST(log_level_from_env_invalid_ignored);