diff --git a/docs/python-pyc.md b/docs/python-pyc.md
index b68c74bf7a..92e1bcc43d 100644
--- a/docs/python-pyc.md
+++ b/docs/python-pyc.md
@@ -42,8 +42,8 @@ even when that owner reaches it again through the same container. Relationships
 are collected after the marshal graph is complete, so a container that a nested
 code object also references is not summarized while its items are still missing.
 Each object reports name, optional qualname,
-filename,
-argument counts, flags, names, locals (or `localsplusnames` on 3.11+), const
+filename, the signed 32-bit `co_firstlineno` value, argument counts, flags,
+names, locals (or `localsplusnames` on 3.11+), const
 summaries, and bytecode length plus SHA-256.
 
 Bytecode is not decompiled. Const strings may be truncated in the summary while
diff --git a/src/domain/python/pythonPyc.ts b/src/domain/python/pythonPyc.ts
index e5415a89cc..eed477c998 100644
--- a/src/domain/python/pythonPyc.ts
+++ b/src/domain/python/pythonPyc.ts
@@ -8,6 +8,7 @@ export const PYTHON_PYC_CHECK_SOURCE = 0x2;
 
 const uint = z.number().int().min(0).max(0xffffffff);
 const index = z.number().int().nonnegative();
+const signedInt32 = z.number().int().min(-0x80000000).max(0x7fffffff);
 /** Non-negative CPython `int` counts. `0xffffffff` is -1 and is not evidence. */
 const signedCount = z.number().int().min(0).max(0x7fffffff);
 
@@ -137,7 +138,7 @@ const pythonConstSchema = z.discriminatedUnion("kind", [
   z.strictObject({
     kind: z.literal("code"),
     name: z.string(),
-    firstlineno: uint,
+    firstlineno: signedInt32,
   }),
   z.strictObject({ kind: z.literal("tuple"), length: uint }),
   z.strictObject({ kind: z.literal("list"), length: uint }),
@@ -157,7 +158,7 @@ const pythonCodeFields = {
   name: z.string(),
   qualname: z.string().nullable(),
   filename: z.string(),
-  firstlineno: uint,
+  firstlineno: signedInt32,
   argcount: signedCount,
   posonlyargcount: signedCount.nullable(),
   kwonlyargcount: signedCount,
diff --git a/src/python/PythonPyc.fixture.ts b/src/python/PythonPyc.fixture.ts
index 550ba51b90..370b8dfda5 100644
--- a/src/python/PythonPyc.fixture.ts
+++ b/src/python/PythonPyc.fixture.ts
@@ -8,6 +8,11 @@ const u32 = (value: number): Buffer => {
   bytes.writeUInt32LE(value);
   return bytes;
 };
+const i32 = (value: number): Buffer => {
+  const bytes = Buffer.alloc(4);
+  bytes.writeInt32LE(value);
+  return bytes;
+};
 const concat = (...parts: Buffer[]): Buffer => Buffer.concat(parts);
 
 /** 16-byte CPython 3.7+ pyc header. Hash-based pycs store an 8-byte source hash. */
@@ -136,7 +141,7 @@ export const marshalCode = (
         marshalShortAscii(fields.qualname ?? fields.name ?? "<module>"),
     );
   parts.push(
-    u32(fields.firstlineno ?? 1),
+    i32(fields.firstlineno ?? 1),
     marshalBytes(fields.linetable ?? Buffer.alloc(0)),
   );
   if (layout === "py311")
diff --git a/src/python/PythonPycParser.test.ts b/src/python/PythonPycParser.test.ts
index a2fce3a085..bde2ad727e 100644
--- a/src/python/PythonPycParser.test.ts
+++ b/src/python/PythonPycParser.test.ts
@@ -72,6 +72,46 @@ it("reads 3.7, 3.10 and 3.12 layouts including nested code", () => {
   );
 });
 
+it("preserves signed co_firstlineno in code objects and code constants", () => {
+  const nested = marshalCode("py311", {
+    name: "inner",
+    firstlineno: -7,
+  });
+  const bytes = Buffer.concat([
+    pythonPycHeader(3531),
+    marshalCode("py311", {
+      firstlineno: -1,
+      consts: [nested],
+    }),
+  ]);
+  const report = parse(bytes);
+  expect(root(report).firstlineno).toBe(-1);
+  expect(root(report).consts).toContainEqual({
+    kind: "code",
+    name: "inner",
+    firstlineno: -7,
+  });
+  expect(pythonPycSchema.safeParse(report).success).toBe(true);
+  expect(
+    pythonPycSchema.safeParse({
+      ...report,
+      code_objects: [
+        { ...root(report), firstlineno: -0x8000_0001 },
+        ...report.code_objects.slice(1),
+      ],
+    }).success,
+  ).toBe(false);
+  expect(
+    pythonPycSchema.safeParse({
+      ...report,
+      code_objects: [
+        { ...root(report), firstlineno: 0x8000_0000 },
+        ...report.code_objects.slice(1),
+      ],
+    }).success,
+  ).toBe(false);
+});
+
 it("decodes a hash-based header and a FLAG_REF top-level code object", () => {
   const bytes = pythonPycFixture("py311", {
     flaggedCode: true,
diff --git a/src/python/PythonPycParser.ts b/src/python/PythonPycParser.ts
index 03b79bddab..acfe4e1365 100644
--- a/src/python/PythonPycParser.ts
+++ b/src/python/PythonPycParser.ts
@@ -457,7 +457,7 @@ class MarshalReader {
       this.layout === "py311"
         ? asString(this.readObject(depth + 1), "co_qualname")
         : null;
-    const firstlineno = this.u32();
+    const firstlineno = this.i32();
     const linetable = asBytes(this.readObject(depth + 1), "co_linetable");
     const exceptiontable =
       this.layout === "py311"
diff --git a/tests/acceptance/pythonPycCli.test.ts b/tests/acceptance/pythonPycCli.test.ts
index f10e78ce16..579b036d91 100644
--- a/tests/acceptance/pythonPycCli.test.ts
+++ b/tests/acceptance/pythonPycCli.test.ts
@@ -2,7 +2,11 @@ import { mkdir, symlink, writeFile } from "node:fs/promises";
 import { join } from "node:path";
 import { expect } from "vitest";
 import { toolContract } from "../../src/contracts/toolContracts.js";
-import { pythonPycFixture } from "../../src/python/PythonPyc.fixture.js";
+import {
+  marshalCode,
+  pythonPycFixture,
+  pythonPycHeader,
+} from "../../src/python/PythonPyc.fixture.js";
 import { createTestTempDirectory } from "../fixtures/temporaryDirectory.js";
 import { cliTest } from "../support/cli/cliFixture.js";
 
@@ -24,6 +28,39 @@ cliTest(
   },
 );
 
+cliTest(
+  "preserves negative CPython first line numbers through the public CLI",
+  async ({ cli }) => {
+    const root = await createTestTempDirectory("rea-python-pyc-lines-");
+    const path = join(root, "sample.pyc");
+    const nested = marshalCode("py311", {
+      name: "inner",
+      firstlineno: -7,
+    });
+    const bytes = Buffer.concat([
+      pythonPycHeader(3531),
+      marshalCode("py311", {
+        firstlineno: -1,
+        consts: [nested],
+      }),
+    ]);
+    await writeFile(path, bytes);
+    const result = await cli.run({
+      arguments: ["inspect-python-pyc", path, "--json"],
+    });
+    expect(result.exitCode, result.stderr).toBe(0);
+    const evidence = toolContract("inspect_python_pyc").outputSchema.parse(
+      result.json,
+    );
+    expect(evidence.normalized_result.code_objects[0]?.firstlineno).toBe(-1);
+    expect(evidence.normalized_result.code_objects[0]?.consts).toContainEqual({
+      kind: "code",
+      name: "inner",
+      firstlineno: -7,
+    });
+  },
+);
+
 cliTest(
   "resolves a relative pyc path against the working directory",
   async ({ cli }) => {
diff --git a/tests/boundary/mcp/pythonPycMcp.test.ts b/tests/boundary/mcp/pythonPycMcp.test.ts
index 07d13da005..c8a70cb31c 100644
--- a/tests/boundary/mcp/pythonPycMcp.test.ts
+++ b/tests/boundary/mcp/pythonPycMcp.test.ts
@@ -1,11 +1,16 @@
 import { writeFile } from "node:fs/promises";
 import { join } from "node:path";
+import { Ajv2020 } from "ajv/dist/2020.js";
 import { expect } from "vitest";
 import { toolContract } from "../../../src/contracts/toolContracts.js";
 import { createServer } from "../../../src/server/createServer.js";
 import { createTestBinarySession } from "../../fixtures/binarySession.js";
 import { parseMcpToolError } from "../../fixtures/mcpToolError.js";
-import { pythonPycFixture } from "../../../src/python/PythonPyc.fixture.js";
+import {
+  marshalCode,
+  pythonPycFixture,
+  pythonPycHeader,
+} from "../../../src/python/PythonPyc.fixture.js";
 import { createTestTempDirectory } from "../../fixtures/temporaryDirectory.js";
 import { mcpTest, type TestMcp } from "../../support/mcp/mcpFixture.js";
 
@@ -29,9 +34,27 @@ mcpTest(
         readOnlyHint: false,
         idempotentHint: true,
       });
+      if (advertised?.outputSchema === undefined)
+        throw new Error("Missing inspect_python_pyc output schema");
+      const outputSchema: Record<string, unknown> = advertised.outputSchema;
+      const ajv = new Ajv2020({ strict: false, validateFormats: false });
+      expect(ajv.validateSchema(outputSchema)).toBe(true);
       const root = await createTestTempDirectory("rea-python-pyc-mcp-");
       const path = join(root, "sample.pyc");
-      await writeFile(path, pythonPycFixture("py311", { nested: true }));
+      const nested = marshalCode("py311", {
+        name: "inner",
+        firstlineno: -7,
+      });
+      await writeFile(
+        path,
+        Buffer.concat([
+          pythonPycHeader(3531),
+          marshalCode("py311", {
+            firstlineno: -1,
+            consts: [nested],
+          }),
+        ]),
+      );
       const response = await client.callTool({
         name: "inspect_python_pyc",
         arguments: { path },
@@ -40,8 +63,20 @@ mcpTest(
       const evidence = toolContract("inspect_python_pyc").outputSchema.parse(
         response.structuredContent,
       );
+      expect(
+        ajv.validate(outputSchema, response.structuredContent),
+        JSON.stringify(ajv.errors),
+      ).toBe(true);
       expect(evidence.normalized_result.header.python_version).toBe("3.12");
+      expect(evidence.normalized_result.code_objects[0]?.firstlineno).toBe(-1);
       expect(evidence.normalized_result.code_objects[1]?.name).toBe("inner");
+      expect(evidence.normalized_result.code_objects[0]?.consts).toContainEqual(
+        {
+          kind: "code",
+          name: "inner",
+          firstlineno: -7,
+        },
+      );
     } finally {
       await session.close();
     }