diff --git a/packages/opencode/src/mcp/discover.ts b/packages/opencode/src/mcp/discover.ts
index eef0d97236..d56312e952 100644
--- a/packages/opencode/src/mcp/discover.ts
+++ b/packages/opencode/src/mcp/discover.ts
@@ -117,6 +117,18 @@ async function readJsonSafe(filePath: string): Promise<any | undefined> {
   } catch {
     return undefined
   }
+  // altimate_change start — apply env-var interpolation to external MCP configs
+  // External configs (Claude Code, Cursor, Copilot, Gemini) may contain ${VAR}
+  // or {env:VAR} references that need resolving before JSON parsing.
+  // Uses ConfigPaths.parseText which runs substitute() then parses JSONC.
+  try {
+    const { ConfigPaths } = await import("../config/paths")
+    return await ConfigPaths.parseText(text, filePath, "empty")
+  } catch {
+    // Substitution or parse failure — fall back to direct parse without interpolation
+    log.debug("env-var interpolation failed for external MCP config, falling back to direct parse", { file: filePath })
+  }
+  // altimate_change end
   const errors: any[] = []
   const result = parseJsonc(text, errors, { allowTrailingComma: true })
   if (errors.length > 0) {
diff --git a/packages/opencode/src/mcp/index.ts b/packages/opencode/src/mcp/index.ts
index 4c80b0ef83..4042b984d0 100644
--- a/packages/opencode/src/mcp/index.ts
+++ b/packages/opencode/src/mcp/index.ts
@@ -25,6 +25,29 @@ import { TuiEvent } from "@/cli/cmd/tui/event"
 import open from "open"
 import { Telemetry } from "@/telemetry"
 
+// altimate_change start — resolve env-var references in MCP environment values
+// Handles ${VAR}, ${VAR:-default}, and {env:VAR} patterns that may have survived
+// config parsing (e.g. discovered external configs, config updates via updateGlobal).
+const ENV_VAR_PATTERN =
+  /\$\$(\{[A-Za-z_][A-Za-z0-9_]*(?::-[^}]*)?\})|(?<!\$)\$\{([A-Za-z_][A-Za-z0-9_]*)(?::-([^}]*))?\}|\{env:([^}]+)\}/g
+
+export function resolveEnvVars(environment: Record<string, string>): Record<string, string> {
+  const resolved: Record<string, string> = {}
+  for (const [key, value] of Object.entries(environment)) {
+    resolved[key] = value.replace(ENV_VAR_PATTERN, (match, escaped, dollarVar, dollarDefault, braceVar) => {
+      if (escaped !== undefined) return "$" + escaped
+      if (dollarVar !== undefined) {
+        const envValue = process.env[dollarVar]
+        return envValue !== undefined && envValue !== "" ? envValue : (dollarDefault ?? "")
+      }
+      if (braceVar !== undefined) return process.env[braceVar] || ""
+      return match
+    })
+  }
+  return resolved
+}
+// altimate_change end
+
 export namespace MCP {
   const log = Log.create({ service: "mcp" })
   const DEFAULT_TIMEOUT = 30_000
@@ -509,7 +532,9 @@ export namespace MCP {
         env: {
           ...process.env,
           ...(cmd === "altimate" || cmd === "altimate-code" ? { BUN_BE_BUN: "1" } : {}),
-          ...mcp.environment,
+          // altimate_change start — resolve ${VAR} / {env:VAR} patterns that survived config parsing
+          ...(mcp.environment ? resolveEnvVars(mcp.environment) : {}),
+          // altimate_change end
         },
       })
       transport.stderr?.on("data", (chunk: Buffer) => {
diff --git a/packages/opencode/test/mcp/env-var-interpolation.test.ts b/packages/opencode/test/mcp/env-var-interpolation.test.ts
new file mode 100644
index 0000000000..a79ef40be5
--- /dev/null
+++ b/packages/opencode/test/mcp/env-var-interpolation.test.ts
@@ -0,0 +1,209 @@
+// altimate_change start — tests for MCP env-var interpolation (closes #656)
+import { describe, test, expect, beforeEach, afterEach } from "bun:test"
+import { mkdir, writeFile } from "fs/promises"
+import path from "path"
+import { resolveEnvVars } from "../../src/mcp"
+import { tmpdir } from "../fixture/fixture"
+
+// -------------------------------------------------------------------------
+// resolveEnvVars — safety-net resolver at MCP launch site
+// -------------------------------------------------------------------------
+
+describe("resolveEnvVars", () => {
+  const ORIGINAL_ENV = { ...process.env }
+
+  beforeEach(() => {
+    process.env["TEST_TOKEN"] = "secret-123"
+    process.env["TEST_HOST"] = "gitlab.example.com"
+  })
+
+  afterEach(() => {
+    process.env = { ...ORIGINAL_ENV }
+  })
+
+  test("resolves ${VAR} syntax", () => {
+    const result = resolveEnvVars({
+      API_TOKEN: "${TEST_TOKEN}",
+      HOST: "${TEST_HOST}",
+    })
+    expect(result.API_TOKEN).toBe("secret-123")
+    expect(result.HOST).toBe("gitlab.example.com")
+  })
+
+  test("resolves {env:VAR} syntax", () => {
+    const result = resolveEnvVars({
+      API_TOKEN: "{env:TEST_TOKEN}",
+    })
+    expect(result.API_TOKEN).toBe("secret-123")
+  })
+
+  test("resolves ${VAR:-default} with fallback when unset", () => {
+    const result = resolveEnvVars({
+      MODE: "${UNSET_VAR:-production}",
+    })
+    expect(result.MODE).toBe("production")
+  })
+
+  test("resolves ${VAR:-default} to env value when set", () => {
+    const result = resolveEnvVars({
+      TOKEN: "${TEST_TOKEN:-fallback}",
+    })
+    expect(result.TOKEN).toBe("secret-123")
+  })
+
+  test("preserves $${VAR} as literal ${VAR}", () => {
+    const result = resolveEnvVars({
+      TEMPLATE: "$${TEST_TOKEN}",
+    })
+    expect(result.TEMPLATE).toBe("${TEST_TOKEN}")
+  })
+
+  test("resolves unset variable to empty string", () => {
+    const result = resolveEnvVars({
+      MISSING: "${COMPLETELY_UNSET_VAR_XYZ}",
+    })
+    expect(result.MISSING).toBe("")
+  })
+
+  test("passes through plain values without modification", () => {
+    const result = resolveEnvVars({
+      PLAIN: "just-a-string",
+      URL: "https://gitlab.com/api/v4",
+    })
+    expect(result.PLAIN).toBe("just-a-string")
+    expect(result.URL).toBe("https://gitlab.com/api/v4")
+  })
+
+  test("resolves multiple variables in a single value", () => {
+    const result = resolveEnvVars({
+      URL: "https://${TEST_HOST}/api?token=${TEST_TOKEN}",
+    })
+    expect(result.URL).toBe("https://gitlab.example.com/api?token=secret-123")
+  })
+
+  test("handles mixed resolved and plain entries", () => {
+    const result = resolveEnvVars({
+      TOKEN: "${TEST_TOKEN}",
+      STATIC_URL: "https://gitlab.com/api/v4",
+      HOST: "{env:TEST_HOST}",
+    })
+    expect(result.TOKEN).toBe("secret-123")
+    expect(result.STATIC_URL).toBe("https://gitlab.com/api/v4")
+    expect(result.HOST).toBe("gitlab.example.com")
+  })
+
+  test("does not interpolate bare $VAR without braces", () => {
+    const result = resolveEnvVars({
+      TOKEN: "$TEST_TOKEN",
+    })
+    expect(result.TOKEN).toBe("$TEST_TOKEN")
+  })
+
+  test("handles empty environment object", () => {
+    const result = resolveEnvVars({})
+    expect(result).toEqual({})
+  })
+})
+
+// -------------------------------------------------------------------------
+// Discovery integration — env vars in external MCP configs
+// -------------------------------------------------------------------------
+
+describe("discoverExternalMcp with env-var interpolation", () => {
+  const ORIGINAL_ENV = { ...process.env }
+
+  beforeEach(() => {
+    process.env["TEST_MCP_TOKEN"] = "glpat-secret-token"
+    process.env["TEST_MCP_HOST"] = "https://gitlab.internal.com"
+  })
+
+  afterEach(() => {
+    process.env = { ...ORIGINAL_ENV }
+  })
+
+  test("resolves ${VAR} in discovered .vscode/mcp.json environment", async () => {
+    await using tmp = await tmpdir()
+    const dir = tmp.path
+    await mkdir(path.join(dir, ".vscode"), { recursive: true })
+    await writeFile(
+      path.join(dir, ".vscode/mcp.json"),
+      JSON.stringify({
+        servers: {
+          gitlab: {
+            command: "node",
+            args: ["gitlab-server.js"],
+            env: {
+              GITLAB_TOKEN: "${TEST_MCP_TOKEN}",
+              GITLAB_HOST: "${TEST_MCP_HOST}",
+              STATIC_VALUE: "no-interpolation-needed",
+            },
+          },
+        },
+      }),
+    )
+
+    const { discoverExternalMcp } = await import("../../src/mcp/discover")
+    const { servers } = await discoverExternalMcp(dir)
+
+    expect(servers["gitlab"]).toBeDefined()
+    expect(servers["gitlab"].type).toBe("local")
+    const env = (servers["gitlab"] as any).environment
+    expect(env.GITLAB_TOKEN).toBe("glpat-secret-token")
+    expect(env.GITLAB_HOST).toBe("https://gitlab.internal.com")
+    expect(env.STATIC_VALUE).toBe("no-interpolation-needed")
+  })
+
+  test("resolves {env:VAR} in discovered .cursor/mcp.json environment", async () => {
+    await using tmp = await tmpdir()
+    const dir = tmp.path
+    await mkdir(path.join(dir, ".cursor"), { recursive: true })
+    await writeFile(
+      path.join(dir, ".cursor/mcp.json"),
+      JSON.stringify({
+        mcpServers: {
+          "my-tool": {
+            command: "npx",
+            args: ["-y", "my-mcp-tool"],
+            env: {
+              API_KEY: "{env:TEST_MCP_TOKEN}",
+            },
+          },
+        },
+      }),
+    )
+
+    const { discoverExternalMcp } = await import("../../src/mcp/discover")
+    const { servers } = await discoverExternalMcp(dir)
+
+    expect(servers["my-tool"]).toBeDefined()
+    const env = (servers["my-tool"] as any).environment
+    expect(env.API_KEY).toBe("glpat-secret-token")
+  })
+
+  test("resolves ${VAR:-default} with fallback in discovered config", async () => {
+    await using tmp = await tmpdir()
+    const dir = tmp.path
+    await mkdir(path.join(dir, ".vscode"), { recursive: true })
+    await writeFile(
+      path.join(dir, ".vscode/mcp.json"),
+      JSON.stringify({
+        servers: {
+          svc: {
+            command: "node",
+            args: ["svc.js"],
+            env: {
+              MODE: "${UNSET_VAR_ABC:-production}",
+            },
+          },
+        },
+      }),
+    )
+
+    const { discoverExternalMcp } = await import("../../src/mcp/discover")
+    const { servers } = await discoverExternalMcp(dir)
+
+    const env = (servers["svc"] as any).environment
+    expect(env.MODE).toBe("production")
+  })
+})
+// altimate_change end