diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index a6c92980..c7b49c79 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -126,3 +126,15 @@ Error messages should never include raw values from sensitive sources like envir
 **Prevention:**
 1.  Avoid including raw values in error messages when the source is potentially sensitive (env vars, auth headers).
 2.  Use generic error messages for validation failures of sensitive data.
+
+## 2026-04-20 - [MEDIUM] Environment Variable Leakage in Configuration Error Messages
+
+**Vulnerability:**
+Several configuration parsers (`EnvConfigParser`, `MCPServer`, and `artifact-signing-config`) were throwing `ConfigurationError` messages that included the raw, invalid values provided by environment variables. For example, `throw new ConfigurationError(\`...got '${entry}'\`);`. If an administrator mistakenly mapped an environment variable containing a sensitive secret (like an API key or a secret threshold) to one of these configuration fields, the invalid value (the secret) would be logged or returned to the user in the error message.
+
+**Learning:**
+Validation and configuration error messages must be careful not to echo back the exact invalid input when that input originates from a source that might contain sensitive data, such as environment variables. A seemingly harmless error message can inadvertently become an information disclosure vector.
+
+**Prevention:**
+1. Do not include raw input values in error messages for configuration parameters derived from environment variables.
+2. State the expected format or type in the error message without reflecting the invalid input.
diff --git a/src/automation/artifact-signing-config.ts b/src/automation/artifact-signing-config.ts
index 1b328b7f..678f0613 100644
--- a/src/automation/artifact-signing-config.ts
+++ b/src/automation/artifact-signing-config.ts
@@ -53,5 +53,5 @@ function parseExplicitBoolean(value: string | undefined, envName: string): boole
     return false;
   }
 
-  throw new ConfigurationError(`${envName} must be either 'true' or 'false', got '${normalized}'.`);
+  throw new ConfigurationError(`${envName} must be either 'true' or 'false'.`);
 }
diff --git a/src/mcp/mcp-server.ts b/src/mcp/mcp-server.ts
index 9e35ac39..e26dfeed 100644
--- a/src/mcp/mcp-server.ts
+++ b/src/mcp/mcp-server.ts
@@ -2483,7 +2483,7 @@ export class MCPServer {
     const parsed = Number(raw);
     if (Number.isNaN(parsed) || parsed <= 0) {
       throw new ConfigurationError(
-        `Invalid MCP4_TOOL_FILTER_WARN_THRESHOLD_PCT: expected positive number, got '${raw}'.`
+        `Invalid MCP4_TOOL_FILTER_WARN_THRESHOLD_PCT: expected positive number.`
       );
     }
     return parsed;
diff --git a/src/tool-filter/config/env-config-parser.ts b/src/tool-filter/config/env-config-parser.ts
index e11a617f..178d3a93 100644
--- a/src/tool-filter/config/env-config-parser.ts
+++ b/src/tool-filter/config/env-config-parser.ts
@@ -100,7 +100,7 @@ export class EnvConfigParser {
       }
 
       throw new ConfigurationError(
-        `MCP4_TOOL_FILTER_ALLOW_CATEGORIES supports only 'list' and 'read', got '${entry}'`
+        `MCP4_TOOL_FILTER_ALLOW_CATEGORIES supports only 'list' and 'read'`
       );
     }