diff --git a/packages/cli/src/context/connections/read-only-sql.ts b/packages/cli/src/context/connections/read-only-sql.ts
index cd2ab0b1..1bde80b1 100644
--- a/packages/cli/src/context/connections/read-only-sql.ts
+++ b/packages/cli/src/context/connections/read-only-sql.ts
@@ -1,3 +1,5 @@
+import { KtxQueryError } from '../../errors.js';
+
 const MUTATING_SQL =
   /^\s*(insert|update|delete|merge|alter|drop|create|truncate|grant|revoke|copy|call|do|vacuum|analyze|refresh)\b/i;
 const READ_SQL = /^\s*(select|with)\b/i;
@@ -80,7 +82,7 @@ function assertSingleSqlStatement(sql: string): void {
     if (sql[index] === ';') {
       sawSemicolon = true;
     } else if (sawSemicolon && !/\s/.test(sql[index])) {
-      throw new Error('Only one SQL statement can be executed.');
+      throw new KtxQueryError('Only one SQL statement can be executed.');
     }
     index += 1;
   }
@@ -89,7 +91,7 @@ function assertSingleSqlStatement(sql: string): void {
 export function assertReadOnlySql(sql: string): string {
   const trimmed = stripLeadingSqlComments(sql).trim();
   if (!READ_SQL.test(trimmed) || MUTATING_SQL.test(trimmed)) {
-    throw new Error('Only read-only SELECT/WITH queries can be executed locally.');
+    throw new KtxQueryError('Only read-only SELECT/WITH queries can be executed locally.');
   }
   assertSingleSqlStatement(trimmed);
   return trimmed;
@@ -133,7 +135,7 @@ export function limitSqlForExecution(sql: string, maxRows: number | undefined):
     return trimmed;
   }
   if (!Number.isInteger(maxRows) || maxRows <= 0) {
-    throw new Error('maxRows must be a positive integer.');
+    throw new KtxQueryError('maxRows must be a positive integer.');
   }
   return `select * from (${trimmed}) as ktx_query_result limit ${maxRows}`;
 }
diff --git a/packages/cli/src/context/mcp/context-tools.ts b/packages/cli/src/context/mcp/context-tools.ts
index 71ed3a14..28e7a9c7 100644
--- a/packages/cli/src/context/mcp/context-tools.ts
+++ b/packages/cli/src/context/mcp/context-tools.ts
@@ -10,7 +10,7 @@ import {
   shouldEmitMcpTelemetry,
 } from '../../telemetry/index.js';
 import { collectTelemetryRedactionSecrets } from '../../telemetry/redaction-secrets.js';
-import { scrubErrorClass } from '../../telemetry/scrubber.js';
+import { formatErrorDetail, scrubErrorClass } from '../../telemetry/scrubber.js';
 import type {
   KtxMcpClientInfo,
   KtxMcpContextPorts,
@@ -564,6 +564,28 @@ function clientTelemetryFields(
   };
 }
 
+// Tools registered via registerParsedTool catch their own errors and return an
+// isError result, so the telemetry layer never sees the thrown Error. Recover
+// the failure message from the result's text content (the same string the agent
+// reads) so the outcome event is self-diagnosing.
+function mcpErrorResultDetail(result: unknown): string | undefined {
+  if (typeof result !== 'object' || result === null || !('content' in result)) {
+    return undefined;
+  }
+  const content = (result as { content?: unknown }).content;
+  if (!Array.isArray(content)) {
+    return undefined;
+  }
+  const text = content
+    .map((block) =>
+      typeof block === 'object' && block !== null && typeof (block as { text?: unknown }).text === 'string'
+        ? (block as { text: string }).text
+        : '',
+    )
+    .join('\n');
+  return formatErrorDetail(text);
+}
+
 function instrumentMcpServer(
   server: KtxMcpServerLike,
   telemetry: { projectDir?: string; io?: KtxCliIo; getClientInfo?: () => KtxMcpClientInfo | undefined },
@@ -577,6 +599,7 @@ function instrumentMcpServer(
           if (telemetry.io && telemetry.projectDir && shouldEmitMcpTelemetry()) {
             const isError =
               typeof result === 'object' && result !== null && 'isError' in result && result.isError === true;
+            const errorDetail = isError ? mcpErrorResultDetail(result) : undefined;
             await emitTelemetryEvent({
               name: 'mcp_request_completed',
               projectDir: telemetry.projectDir,
@@ -586,6 +609,7 @@ function instrumentMcpServer(
                 outcome: isError ? 'error' : 'ok',
                 durationMs: Math.max(0, performance.now() - startedAt),
                 sampleRate: mcpTelemetrySampleRate(),
+                ...(errorDetail ? { errorDetail } : {}),
                 ...clientTelemetryFields(telemetry.getClientInfo),
               },
             });
@@ -608,6 +632,7 @@ function instrumentMcpServer(
           }
           if (telemetry.io && telemetry.projectDir && shouldEmitMcpTelemetry()) {
             const errorClass = scrubErrorClass(error);
+            const errorDetail = formatErrorDetail(error);
             await emitTelemetryEvent({
               name: 'mcp_request_completed',
               projectDir: telemetry.projectDir,
@@ -616,6 +641,7 @@ function instrumentMcpServer(
                 toolName: name,
                 outcome: 'error',
                 ...(errorClass ? { errorClass } : {}),
+                ...(errorDetail ? { errorDetail } : {}),
                 durationMs: Math.max(0, performance.now() - startedAt),
                 sampleRate: mcpTelemetrySampleRate(),
                 ...clientTelemetryFields(telemetry.getClientInfo),
diff --git a/packages/cli/src/context/mcp/local-project-ports.ts b/packages/cli/src/context/mcp/local-project-ports.ts
index d3106593..7c2863cb 100644
--- a/packages/cli/src/context/mcp/local-project-ports.ts
+++ b/packages/cli/src/context/mcp/local-project-ports.ts
@@ -1,6 +1,6 @@
 import type { KtxSqlQueryExecutorPort } from '../../context/connections/query-executor.js';
 import { resolveConfiguredConnection } from '../../context/connections/resolve-connection.js';
-import { KtxQueryError, isNativeProgrammingFault } from '../../errors.js';
+import { KtxExpectedError, KtxQueryError, isNativeProgrammingFault } from '../../errors.js';
 import { localConnectionInfoFromConfig } from '../../context/connections/local-warehouse-descriptor.js';
 import type { KtxEmbeddingPort } from '../../context/core/embedding.js';
 import type { KtxSemanticLayerComputePort } from '../../context/daemon/semantic-layer-compute.js';
@@ -46,7 +46,9 @@ async function executeValidatedReadOnlySql(
   }
   const validation = await options.sqlAnalysis.validateReadOnly(input.sql, sqlAnalysisDialectForDriver(connection.driver));
   if (!validation.ok) {
-    throw new Error(validation.error ?? 'SQL is not read-only.');
+    // A read-only guard rejecting the agent's SQL is an expected outcome, not a
+    // ktx fault: classify it so reportException keeps it out of Error Tracking.
+    throw new KtxQueryError(validation.error ?? 'SQL is not read-only.');
   }
   const createConnector = options.localScan?.createConnector;
   if (!createConnector) {
@@ -75,7 +77,7 @@ async function executeValidatedReadOnlySql(
         // while preserving the warehouse's own diagnostics. A native JS error
         // (TypeError, etc.) signals a bug in connector code — let it propagate
         // unchanged so Error Tracking still sees it.
-        if (isNativeProgrammingFault(error)) {
+        if (isNativeProgrammingFault(error) || error instanceof KtxExpectedError) {
           throw error;
         }
         throw new KtxQueryError(error instanceof Error ? error.message : String(error), { cause: error });
diff --git a/packages/cli/src/telemetry/events.schema.json b/packages/cli/src/telemetry/events.schema.json
index c6c3d6f8..405f0240 100644
--- a/packages/cli/src/telemetry/events.schema.json
+++ b/packages/cli/src/telemetry/events.schema.json
@@ -162,6 +162,7 @@
         "outcome",
         "durationMs",
         "errorClass",
+        "errorDetail",
         "sampleRate",
         "mcpClientName",
         "mcpClientVersion"
@@ -1167,6 +1168,10 @@
         "errorClass": {
           "type": "string"
         },
+        "errorDetail": {
+          "type": "string",
+          "maxLength": 1000
+        },
         "sampleRate": {
           "type": "number",
           "const": 1
diff --git a/packages/cli/src/telemetry/events.ts b/packages/cli/src/telemetry/events.ts
index cf650492..6d311ac5 100644
--- a/packages/cli/src/telemetry/events.ts
+++ b/packages/cli/src/telemetry/events.ts
@@ -161,6 +161,7 @@ const mcpRequestCompletedSchema = telemetryCommonEnvelopeSchema
     outcome: outcomeSchema,
     durationMs: z.number().nonnegative(),
     errorClass: z.string().optional(),
+    errorDetail: z.string().max(1000).optional(),
     sampleRate: z.literal(1),
     // Raw, client-tool-controlled identity from the MCP initialize handshake
     // (clientInfo.name/version). Optional: clients may omit clientInfo. Stored
@@ -349,7 +350,16 @@ export const telemetryEventCatalog = [
   {
     name: 'mcp_request_completed',
     description: 'Emitted for sampled MCP tool requests.',
-    fields: ['toolName', 'outcome', 'durationMs', 'errorClass', 'sampleRate', 'mcpClientName', 'mcpClientVersion'],
+    fields: [
+      'toolName',
+      'outcome',
+      'durationMs',
+      'errorClass',
+      'errorDetail',
+      'sampleRate',
+      'mcpClientName',
+      'mcpClientVersion',
+    ],
   },
   {
     name: 'daemon_started',
diff --git a/packages/cli/test/context/connections/read-only-sql.test.ts b/packages/cli/test/context/connections/read-only-sql.test.ts
index 4504a126..8a090c28 100644
--- a/packages/cli/test/context/connections/read-only-sql.test.ts
+++ b/packages/cli/test/context/connections/read-only-sql.test.ts
@@ -1,4 +1,5 @@
 import { describe, expect, it } from 'vitest';
+import { KtxExpectedError, KtxQueryError } from '../../../src/errors.js';
 import {
   assertReadOnlySql,
   limitSqlForExecution,
@@ -20,6 +21,20 @@ describe('assertReadOnlySql', () => {
     );
   });
 
+  // A guard refusing the agent's SQL is an expected outcome; classifying it as
+  // KtxQueryError keeps reportException from filing it as a ktx fault.
+  it('rejects with an expected KtxQueryError, not a bare Error', () => {
+    expect(() => assertReadOnlySql('delete from orders')).toThrow(KtxQueryError);
+    expect(() => assertReadOnlySql('describe orders')).toThrow(KtxQueryError);
+    expect(() => assertReadOnlySql('select 1; drop table orders')).toThrow(KtxQueryError);
+    try {
+      assertReadOnlySql('describe orders');
+      expect.unreachable('expected a throw');
+    } catch (error) {
+      expect(error).toBeInstanceOf(KtxExpectedError);
+    }
+  });
+
   it('accepts read-only queries that begin with leading comments', () => {
     expect(assertReadOnlySql('-- daily widget sales\nselect count(*) from public.widget_sales')).toBe(
       'select count(*) from public.widget_sales',
diff --git a/packages/cli/test/context/mcp/server.test.ts b/packages/cli/test/context/mcp/server.test.ts
index 6b52bd66..96a4bd55 100644
--- a/packages/cli/test/context/mcp/server.test.ts
+++ b/packages/cli/test/context/mcp/server.test.ts
@@ -287,6 +287,35 @@ describe('createKtxMcpServer', () => {
     expect(io.stderrText()).not.toContain('mcpClientVersion');
   });
 
+  it('records the failure message as errorDetail when a tool returns an error', async () => {
+    vi.spyOn(Math, 'random').mockReturnValue(0);
+    vi.stubEnv('KTX_TELEMETRY_DEBUG', '1');
+    vi.stubEnv('CI', '');
+    const fake = makeFakeServer();
+    const io = makeIo();
+
+    createKtxMcpServer({
+      server: fake.server,
+      userContext: { userId: 'local-user' },
+      projectDir: '/tmp/ktx-mcp-error-detail',
+      io,
+      contextTools: {
+        knowledge: {
+          search: vi.fn<KtxKnowledgeMcpPort['search']>().mockRejectedValue(new Error('wiki search exploded')),
+          read: vi.fn<KtxKnowledgeMcpPort['read']>().mockResolvedValue(null),
+        },
+      },
+    });
+
+    await expect(getTool(fake.tools, 'wiki_search').handler({ query: 'revenue', limit: 5 })).resolves.toMatchObject({
+      isError: true,
+    });
+
+    expect(io.stderrText()).toContain('"event":"mcp_request_completed"');
+    expect(io.stderrText()).toContain('"outcome":"error"');
+    expect(io.stderrText()).toContain('"errorDetail":"wiki search exploded"');
+  });
+
   it('reports MCP tool exceptions with a tool-derived source', async () => {
     reportExceptionMock.mockClear();
     vi.stubEnv('ANTHROPIC_API_KEY', 'mcp-anthropic-secret'); // pragma: allowlist secret
diff --git a/python/ktx-daemon/src/ktx_daemon/telemetry/events.schema.json b/python/ktx-daemon/src/ktx_daemon/telemetry/events.schema.json
index c6c3d6f8..405f0240 100644
--- a/python/ktx-daemon/src/ktx_daemon/telemetry/events.schema.json
+++ b/python/ktx-daemon/src/ktx_daemon/telemetry/events.schema.json
@@ -162,6 +162,7 @@
         "outcome",
         "durationMs",
         "errorClass",
+        "errorDetail",
         "sampleRate",
         "mcpClientName",
         "mcpClientVersion"
@@ -1167,6 +1168,10 @@
         "errorClass": {
           "type": "string"
         },
+        "errorDetail": {
+          "type": "string",
+          "maxLength": 1000
+        },
         "sampleRate": {
           "type": "number",
           "const": 1